The Effect of Increasing the Amount of Indium Alloying Material on the Efﬁciency of Sacriﬁcial Aluminium Anodes

materials

Article The Effect of Increasing the Amount of Indium Alloying Material on the Efﬁciency of Sacriﬁcial Aluminium Anodes

Krzysztof Zakowski 1,* , Juliusz Orlikowski 1, Kazimierz Darowicki 1 , Marcin Czekajlo 2, Piotr Iglinski 3 and Kinga Domanska 3

1 Department of Electrochemistry, Corrosion and Materials Engineering, Faculty of Chemistry, Gdansk University of Technology, 11/12 Gabriela Narutowicza Street, 80-233 Gdansk, Poland; [email protected] (J.O.); [email protected] (K.D.) 2 KGHM Polska Miedz S.A. Ore Concentration Plant, 1 Kopalniana Street, 59-101 Polkowice, Poland; [email protected] 3 LOTOS Petrobaltic Ltd., 9 Stary Dwor Street, 80-758 Gdansk, Poland; [email protected] (P.I.); [email protected] (K.D.) * Correspondence: [email protected]

Abstract: Al-Zn-In alloys having 4.2% zinc content and various indium content in the range of 0.02–0.2% were tested with respect to the most important electrochemical properties of sacrificial anodes in a cathodic protection, i.e., the current capacity and potential of the operating anode. The distribution of In and Zn in the tested alloys was mapped by means of the EDX technique, which demonstrated that these elements dissolve well in the alloy matrix and are evenly distributed within it. The current capacity of such alloys was determined by means of the method of determining the mass loss during the dissolution by a current of known charge. The results obtained demonstrate that the current capacity of Al-Zn-In alloy decreases with the increase in the In content, which results Citation: Zakowski, K.; Orlikowski, in an increased consumption of anode material and shorter lifetime of anodes. With 0.02% In content, J.; Darowicki, K.; Czekajlo, M.; the capacity amounted to approx. 2500 Ah/kg, whereas the alloy with 0.2% In had as much as 30% Iglinski, P.; Domanska, K. The Effect lower capacity amounting to approx. 1750 Ah/kg. Microscopic examination for the morphology of Increasing the Amount of Indium and surface profile of the samples after their exposure demonstrated that a higher indium content Alloying Material on the Efficiency of Sacrificial Aluminium Anodes. in the alloy results in a more uneven general corrosion pattern during the dissolution of such alloy, Materials 2021, 14, 1755. https:// and the cavities (pits) appearing on the alloy surface are larger and deeper. As the indium content is doi.org/10.3390/ma14071755 increased from 0.02% to 0.05%, the Al-Zn-In alloy potential decreases by about 50 mV to −1100 mV vs. Ag/AgCl electrode, which is advantageous in terms of using this alloy as a sacrificial anode. Academic Editor: Joan-Josep Suñol When the indium content is further increased from 0.05% to 0.2%, the potential of the alloy is no longer changed to a more negative one. The results obtained from all these tests demonstrate that Received: 5 March 2021 alloys containing up to 0.05% of In additive are practically applicable for cathodic protection. Accepted: 30 March 2021 Published: 2 April 2021 Keywords: Al-Zn-In; aluminium sacrificial anode; aluminium galvanic anode; cathodic protection; current capacity of sacrificial anode; closed circuit potential of sacrificial anode Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Aluminium is the third most abundant element (after oxygen and silicon) on Earth. Aluminium products are used as semi-finished or finished products in practically every industry, e.g., automotive, packaging, construction, chemical, energy, electronics, metal- Copyright: © 2021 by the authors. lurgy, etc. Licensee MDPI, Basel, Switzerland. Aluminium is also used for the cathodic protection, which is one of the main anti- This article is an open access article distributed under the terms and corrosion technologies for underwater and underground steel structures. In the cathodic conditions of the Creative Commons protection (which is based on lowering the potential of the protected structure by means of Attribution (CC BY) license (https:// the current flowing from anodes), it is necessary to provide an amount of the protective creativecommons.org/licenses/by/ current that polarizes the structure up to the protective potential [1]. Thereafter, the 4.0/). corrosion processes are inhibited and the estimated corrosion rate of the structure is less

Materials 2021, 14, 1755. https://doi.org/10.3390/ma14071755 https://www.mdpi.com/journal/materials Materials 2021, 14, 1755 2 of 17

than 0.01 mm/year [2]. Too low a density of the polarizing current makes the structure polarized up to the potential more positive than the protective potential. In such a case, the effect of partial cathodic protection is obtained in which the corrosion processes are partially inhibited, but not completely. Cathodic protection can be implemented by means of two technologies. The first is the ICCP (impressed current cathodic protection system) [3]. The source of the current is a polarizing device—the so-called cathodic protection station supplied from the power grid and cooperating with polarizing anodes which are usually made of non-soluble materials (e.g., titanium and ruthenium coated with MMO—mixed metal oxide [4,5]) or hardly soluble materials (e.g., cast iron [6]). The main advantage of ICCP is the possibility of obtaining a high intensity protection current from anodes, due to a possible high output voltage from the device, which amounts up to 50 V for underground structures and up to 24 V for offshore structures. Hence, a small number of anodes necessary to polarize the entire surface of the protected structure will be sufficient [7]. The second cathodic protection technology is an application of sacrificial anodes (galvanic anodes) directly connected to the protected structure [8,9]. These anodes are made of metals with a more negative potential than the protected structure, which constitute a galvanic cell altogether with the structure. In this cell, anodes are dissolved and supply the cathodic protection current which flows from anodes through the electrolyte (water, earth) to the protected structure. Offshore steel structures are usually protected with aluminium or zinc anodes. Magnesium anodes are used to protect underground structures, although it is also possible to use them to protect offshore structures [10,11]. The potential difference between the operating sacrificial anode and the protected steel structure is small, only approx. 0.25 V, thus the current intensity from the galvanic anode is relatively small. Therefore, a large number of sacrificial anodes is required for offshore structures in order to provide the required amount of cathodic protection current, particularly when the barrier properties of the structure’s protective coating are poor [12,13]. A particular feature of sacrificial anodes is the fact they are dissolved during their operation and it is periodically necessary to replace them with new ones. Another related aspect is the possibility of contaminating the natural environment with anode dissolution products. Research in this field demonstrated that within the area of operating anodes the concentration of aluminium or zinc in seawater increases only slightly; on the other hand, it is high in sea mud in the direct neighbourhood of operating anodes [14,15]. An oxide layer is formed on pure aluminium, which has a passivating effect [16]. A particular improvement in the electrochemical properties of aluminium anodes is achieved by means of the respective alloying additives. The purpose of particular additives is to acti- vate the passivated Al surface, the purpose of other additives is to lower the anode alloy’s potential, whereas the purpose of further additives is to increase the current efficiency [17]. Zn is primarily used for the activation of the passivated aluminium surface, and it is usually added in the amount of 1–5% [18]. The alloying additives such as In, Cd, Ga, Sn, Hg, Mg are also beneficial, because they act as aluminium depasivators [19]. On the other hand, when the amount of Sn is increased above 0.2%, similarly to Mg above 1%, the dissolution rate of aluminium anodes is adversely increased [20]. Zn and In change the anode potential to a more negative one and improve the microstructural uniformity of anodes—the latter leads to more uniform corrosion [21]. Cu alloying additive, which is added to increase the current efficiency, unfortunately leads to the Al2Cu intermetallic phase at grain boundaries, which results in the uneven anode dissolution [22]. Therefore, Sm is added in order to modify the shape of Al2Cu, making it finer and making the corrosion of Al–Zn–Cu alloy uniform [23]. Pitting and crevice corrosion occurring on the surface of Al–5Zn alloy can be limited by adding 2% Mg, 0.15% Mn and 0.02% In, which is evidenced by microstructure examinations of the corroded alloy surfaces with different contents of these additives [24]. The properties of aluminium anodes are adversely affected by impurities, especially Fe and Si, which increase the corrosion rate of Al–Zn alloys and adversely increase the potential of operating anode to a more positive value [25]. Materials 2021, 14, 1755 3 of 17

As discussed above, the scientific literature describes the influence of In on the activation of the passivated surface of aluminium alloys, on the reduction in their self-corrosion and on the OCP value (open circuit potential, which is the corrosive potential of non- operating anode). In terms of the efficiency and operation of the cathodic protection systems, the influence of this element on other properties is very important, particularly the influence on the anode alloy efficiency, its current capacity, as well as electronegative CCP (closed circuit potential) value of the operating anode, despite its dissolution. Large current capacity contributes to the extended lifetime of the anode, i.e., the time it is dissolved, leading to the economic effect. The high electronegative potential of the operating anode enables the polarisation of the protected structure up to the protective potential, even when anodes are covered with corrosion by-products and marine organisms [26,27], or when there is an increased demand for cathodic protection current after the stormy period during which stormy sea water destroys the non-conductive calcareous deposits existing on the protected structure (deposits reduce the demand for cathodic protection current, similarly to the protective coatings [28]). Murray and Morton [29] examined the electrochemical properties of Al–Zn–In alloys with low In content, in a 3% NaCl solution. The results obtained demonstrated that the alloy with 0.02% In content had the best parameters in terms of the CCP potential and the dissolution rate (which is reciprocal to the current capacity). Similar results were obtained by Keyvani, Saremi and Saeri, who tested Al-Zn-In alloys with 1–6% Zn and 0.01–0.05% In [21]. Research centres around the world are constantly examining the influence of various additives on the properties of galvanic anode alloys applicable for cathodic protection, in order to identify the alloys with the best electrochemical properties and extend the service life of anodes. The purpose of the research in this publication is the determination of the effect of an increased amount of indium, in the range of 0.02–0.20%, on the current capacity and the CCP potential of Al-Zn-In sacrificial anode alloys. These electrochemical properties are very important from a practical point of view for designers and users of cathodic protection installations for marine structures. The novelty of the presented work in comparison with the literature are: (i) a wide range of indium addition amounts was tested (for example, in work [21] the content of 0.01–0.05% was tested), (ii) 3D images were used together with the height profiles and the roughness coefficient of the samples surface to explain the cause of changes in the current capacity of the tested alloys, (iii) a hypothesis explaining the mechanism of an observed behaviour of the alloys depending on the indium content was proposed.

2. Materials The tests were carried out on the samples of prepared Al-Zn-In alloys containing the same zinc content and a different indium content. It was assumed that the amount of indium in the tested alloys will be in the range of 0.02–0.20%, by weight. The selected zinc content in alloys amounted to 4.2%, which is the average level used in the commercial manufacturing of aluminium anodes applicable for the cathodic protection of offshore structures, where the zinc content is usually in the range of 3.0–5.5%. The alloys for tests were prepared as follows: A base alloy was prepared, in which the calculated Zn content amounted to 4.2% and the In content amounted to 0.02%. The alloying elements were melted in an electric laboratory furnace in a silicon carbide crucible. The samples were cast from a portion of the base alloy—these were the samples with the smallest amount of indium. Then, the calculated amounts of indium were added to the base alloy, and samples containing more and more indium were cast from the subsequently obtained alloys. In this way, six alloys were obtained in total, in which the Zn content and impurities were practically constant, while the In content increased. Chemical composition of the samples was determined by means of the emission spectrometer. The content of impurities in the base alloy amounted to: Cd < 0.002%, Si < 0.12%, Fe < 0.08%, Cu < 0.003%. Materials 2021, 14, x FOR PEER REVIEW 4 of 17

content of impurities in the base alloy amounted to: Cd < 0.002%, Si < 0.12%, Fe < 0.08%, Cu < 0.003%. The content of various components in the finally obtained alloys, determined by spectrometry, is presented in Table 1.

Table 1. The percentage content of the elements in the tested Al-Zn-In alloys (% by weight). Materials 2021, 14, 1755 4 of 17 Alloy Number Zn In Al 1 4.24 0.022 balance The content2 of various components4.19 in the ﬁnally obtained0.034 alloys, determined by balance spectrometry, is presented in Table1. 3 4.12 0.048 balance Table 1. The percentage4 content of the elements4.22 in the tested Al-Zn-In alloys0.088 (% by weight). balance

Alloy Number5 Zn4.12 In0.133 Al balance 16 4.244.17 0.0220.197 balance balance 2 4.19 0.034 balance 3 4.12 0.048 balance The4 samples for SEM 4.22microscopic tests and 0.088 the samples for balance electrochemical tests were cut from5 the obtained alloy 4.12 ingots. The sample 0.133 preparation method balance for particular types of tests is described6 in the following 4.17 sections. 0.197 balance

3. MethodsThe samples for SEM microscopic tests and the samples for electrochemical tests were cut from the obtained alloy ingots. The sample preparation method for particular types of tests3.1. isMethodology described in theof Microscopic following sections. Research The microscopic tests included: (a) determining the distribution of alloying elements 3. Methods 3.1.in the Methodology tested alloys, of Microscopic (b) examining Research the morphology of the samples after their electrochem- icalThe exposure. microscopic tests included: (a) determining the distribution of alloying elements in the testedAd. (a) alloys, The (b) distribution examining the of morphology alloying elements of the samples in the after tested their samples electrochemi- of Al-Zn-In alloys calwas exposure. examined by means of the HITACHI S-3400N Scanning Electron Microscope (manufacturer:Ad. (a) Hitachi The distribution High ofTechnologies alloying elements America in the tested Inc., samples Schaumburg, of Al-Zn-In IL, alloys USA) and the Ul- was examined by means of the HITACHI S-3400N Scanning Electron Microscope (manu- traDry X-ray detector, with the NSS 312 X-ray microanalysis system (manufacturer: facturer: Hitachi High Technologies America Inc., Schaumburg, IL, USA) and the UltraDry X-rayThermo detector, Fisher with Scientific, the NSS 312 Waltham, X-ray microanalysis MA, US systemA). The (manufacturer: EDS (energy-dispersive Thermo Fisher X-ray spec- Scientiﬁc,troscopy) Waltham, method MA, was USA). used The to EDS map (energy-dispersive the selected sample X-ray spectroscopy) area and to method present the distribu- wastion used of elements to map the on selected the tested sample sample area and surface. to present the distribution of elements on the testedThe sample tested surface. alloy samples with an area of about 1 cm2 were prepared for SEM testing The tested alloy samples with an area of about 1 cm2 were prepared for SEM testing byby polishing polishing the the surface. surface. The samples The samples were polished were bypo 600-grit,lished by 2000-grit 600-grit, and 5000-grit2000-grit and 5000-grit sandpapersandpaper and and then then polished polished with 3 µ withm diamond 3 μm paste.diamond The view paste. of an The exemplary view of sample, an exemplary sam- whichple, which was prepared was prepared for testing, for is presented testing, inis Figurepresented1. in Figure 1.

FigureFigure 1. 1.An An example example sample sample prepared prepared for SEM tests.for SEM tests.

Ad. (b) The surface morphology of samples after completion of the electrochemical tests wasAd. examined (b) The by surface means ofmorphology the Nikon MA200 of samples metallurgical after optical completion microscope of (man-the electrochemical ufacturer:tests was NIKON examined CORRPORATION, by means of the Tokyo, Nikon Japan). MA In200 addition metallurgical to the pictures optical of themicroscope (man- sampleufacturer: surfaces, NIKON a computer CORRPORATION, analysis for the surface Tokyo, proﬁle Japan) was also. In performed,addition andto the the pictures of the roughnesssample surfaces, coefﬁcient a was computer determined. analysis for the surface profile was also performed, and the roughness coefficient was determined.

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3.2. Methodology of Electrochemical Research 3.2. Methodology of Electrochemical Research The current capacity for the prepared alloys was determined by means of the sample mass Theloss currentmethod, capacity which forwas the caused prepared by the alloys outflow was determined of an electric by meanscharge ofof thea known sample mass loss method, which was caused by the outflow of an electric charge of a known value. The method of the 4-day electrochemical test was implemented in accordance with value. The method of the 4-day electrochemical test was implemented in accordance the DNVGL-RP-B401 standard [30]. In this test, the current flow from the anode to the with the DNVGL-RP-B401 standard [30]. In this test, the current flow from the anode auxiliary steel cathode is forced by a galvanostat. Application of the galvanostat facilitates to the auxiliary steel cathode is forced by a galvanostat. Application of the galvanostat the application of precisely defined current loads for the anode during tests, with different facilitates the application of precisely defined current loads for the anode during tests, intensities on individual days, which are much greater than during the normal operation with different intensities on individual days, which are much greater than during the of the anode in cathodic protection installations. Therefore, the test is very “demanding” normal operation of the anode in cathodic protection installations. Therefore, the test is for the anode material. Various loads on the sacrificial anodes also occur during their op- very “demanding” for the anode material. Various loads on the sacrificial anodes also occur eration in real offshore conditions, e.g.,: (i) an increased current demand of the protected during their operation in real offshore conditions, e.g.,: (i) an increased current demand of structure during its initial cathodic polarisation, from the corrosion potential to the pro- the protected structure during its initial cathodic polarisation, from the corrosion potential tective potential, (ii) standard working conditions of sacrificial anodes (the so-called op- to the protective potential, (ii) standard working conditions of sacrificial anodes (the so- erationalcalled operational density of densitypolarising of polarisingcurrent), (iii) current), an increased (iii) an density increased of polarising density of current polarising af- tercurrent the stormy after the period stormy and period during and the during depolari thesation depolarisation of the protected of the structure, protected structure,which is relatedwhich to is relatedthe destruction to the destruction of sediment ofs sediments on the structure on the structureby a stormy by sea. a stormy sea.

3.3.1.3.2.1. Method Method of of Preparing Preparing the the Samples Samples for for Electrochemical Electrochemical Tests Tests TheThe samples samples for for electrochemical electrochemical tests tests were were prepared prepared in in a a cylindrical cylindrical shape shape with with a a diameterdiameter of of 10 10 mm mm and and a aheight height of of 50 50 mm. mm. In In the the base base of of the the cylinder, cylinder, a a hole hole was was drilled drilled andand threaded, threaded, into into which which a athreaded threaded steel steel wire wire with with a diameter a diameter of 0.5 of 0.5cm cmand and a length a length of 20of cm 20 cmwas was screwed, screwed, which which was was used used to tofix ﬁx the the sample sample in in the the measuring measuring vessel vessel and and to to connectconnect the the electrical electrical wires wires to to the the sample. sample. The The wire wire was was secured secured along along its its entire entire length length withwith a a plastic plastic heat-shrinkable heat-shrinkable tubing, tubing, in in order order to to prevent prevent it it from from coming coming into into contact contact with with thethe electrolyte electrolyte in in the the measuring measuring vessel. vessel. The The conne connectingcting point point for for the the wire wire to to the the test test sample sample waswas secured secured with epoxy resin,resin, whichwhich protected protected it it against against the the penetration penetration of electrolyteof electrolyte into intothe the connecting connecting point point and and protected protected the connectionthe connection against against corrosion. corrosion. BecauseBecause the the surface surface of of the the tested tested samples samples could could passivate passivate during during their their mechanical mechanical processingprocessing [31], [31], thethe samplesample surface surface was was cleaned cleaned of theof the passive passive layer layer according according to the to NACE the NACETM 0190 TM standard0190 standard [32] immediately [32] immediately before before the exposure,the exposure, by immersingby immersing the the sample sample for ◦ for5min 5 min in ain hot a hot (80 (80C) °C) sodium sodium hydroxide hydroxide solution solution (50 (50 g of g NaOH of NaOH was was dissolved dissolved in 1 in dm 13 dmdistilled3 distilled water). water). In orderIn order to to remove remove the the black black coating coating formed formed during during the the above above activity activity(by-products (by-products of of chemical chemical reaction reaction between between the the alloy alloy and and NaOH) NaOH) from from the the surfaces surfaces of theof thesamples, samples, the the samples samples were were immersed immersed in in a a concentrated concentrated solutionsolution ofof nitric acid (V) (V) for for a a fewfew seconds, seconds, then then rinsed rinsed in indistilled distilled water water and and acetone acetone and and dried dried in a in stream a stream of hot of air hot for air 15for min. 15 min.After Aftercooling cooling down, down, the samples the samples were weighed were weighed to the nearest to the 0.1 nearest mg. The 0.1 mg.appear- The anceappearance of the exemplary of the exemplary samples, samples, prepared prepared for the exposure, for the exposure, is shown is in shown Figure in 2. Figure 2.

(a) (b)

FigureFigure 2. 2. ViewView of of the the Al-Zn-In Al-Zn-In alloy alloy sample sample specim specimensens prepared prepared for for electrochemical electrochemical tests: tests: (a (a) )After After activationactivation of of the the sample sample surfaces surfaces in NaOH solution; ((bb)) AfterAfter cleaningcleaning the the sample sample surfaces surfaces from from black black deposits in HNO3. deposits in HNO3.

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3.2.2.3.3.2. Measuring System TheThe samplessamples werewere exposedexposed inin glassglass vesselsvessels withwith thethe volumevolume ofof 55 dmdm33.. AnAn auxiliaryauxiliary cathodecathode mademade ofof aa sheetsheet ofof S235JR2S235JR2 carboncarbon steelsteel withwith aa thicknessthickness ofof 0.50.5 mmmm waswas attachedattached inin eacheach ofof them.them. TheThe sheetsheet waswas bentbent intointo aa tubetube withwith aa diameterdiameter ofof 2020 cmcm andand aa lengthlength ofof 2020 cm,cm, andand itit waswas placedplaced nextnext toto thethe vesselvessel wall.wall. TheThe anodeanode samplesample forfor testingtesting waswas placedplaced inin thethe centre,centre, insideinside thethe cathode.cathode. TheThe Ag/AgClAg/AgCl reference electrodeelectrode waswas attachedattached nextnext toto thethe testtest samplesample in in such such a a way way that that the the tip tip of of the the electrode electrode was was at distanceat distance of 1 of mm 1 mm from from the samplethe sample surface. surface. Due Due to such to such a close a close arrangement arrangement of the of electrodethe electrode and and the anodethe anode surface, sur- theface, measured the measured value value of anode of anode potential potential during during its polarisation its polarisation included included a negligible a negligible value ofvalue the IRof the ohmic IR ohmic voltage voltage drop. Afterdrop. fixingAfter fixing all the all elements, the elements, the measuring the measuring vessel vessel was filled was withfilled electrolyte. with electrolyte. An unobstructed An unobstructed air access ai wasr access provided was provided to the electrolyte. to the electrolyte. The tests were The carriedtests were out carried at room out temperature. at room temperature. TheThe sourcesource ofof thethe polarisingpolarising currentcurrent waswas thethe galvanostat,galvanostat, typetype 0931(manufacturer:0931(manufacturer: ATLAS-SOLLICH,ATLAS-SOLLICH, Gdansk,Gdansk, Poland).Poland). InIn orderorder toto enableenable thethe experimentexperiment simultaneouslysimultaneously inin severalseveral vessels,vessels, thethe measuringmeasuring systemssystems werewere connectedconnected withwith eacheach otherother byby electricelectric cablescables inin aa series,series, inin thethe followingfollowing manner:manner: outputoutput (+)(+) ofof thethe galvanostatgalvanostat withwith thethe anodeanode inin thethe firstfirst vessel,vessel, thethe cathodecathode inin thethe firstfirst vesselvessel withwith thethe anodeanode inin thethe secondsecond vessel,vessel, etc.,etc., thethe cathodecathode inin thethe lastlast vesselvessel withwith thethe galvanostatgalvanostat output output ( −(−).). The galvanostat maintainedmaintained thethe predeterminedpredetermined currentcurrent valuevalue withwith anan accuracyaccuracyof of± ±0.010.01 mA. A diagramdiagram ofof thethe measuringmeasuring setset isis shownshown inin FigureFigure3 3..

FigureFigure 3. DiagramDiagram of of the the measuring measuring set: set: 1—auxiliary 1—auxiliary cathode, cathode, 2—Ag/AgCl 2—Ag/AgCl reference reference electrode, electrode, 3— 3—measuringmeasuring vessel, vessel, 4—sample 4—sample anode anode alloy alloy under under test, test, 5—electrolyte, 5—electrolyte, 6—galvanostat. 6—galvanostat.

3.2.3.3.3.3. Electrolyte TheThe teststests werewere carriedcarried outout inin syntheticsynthetic seawaterseawater preparedprepared inin accordanceaccordance withwith ASTMASTM D1141-98D1141-98 “Standard“Standard PracticePractice forfor thethe PreparationPreparation ofof SubstituteSubstitute OceanOcean Water”.Water”. ChemicalChemical 3 reagentsreagents inin thethe amountsamounts speciﬁedspecified inin TableTable2 2 were were used used per per every every 10 10 dm dm3 ofof taptap water,water, inin orderorder toto makemake thethe solution.solution.

3 TableTable 2.2. The amount of of reagents reagents per per 10 10 dm dm3 ofof tap tap water, water, in inorder order to toprepare prepare the the synthetic synthetic seawater. seawater.

Reagent Weight Weight (g)(g) NaCl 245.34 Na SO 40.94 Na22SO44 40.94 MgCl2·6H2O 111.24 MgCl2·6H2O 111.24 CaCl2 11.58 2 SrClCaCl2·6H2 O11.58 0.42 SrClKCl2·6H2O 6.950.42 NaHCOKCl 3 2.016.95 NaHCO3 2.01

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3.2.4. Current Load on the Samples during the Exposure Loading the samples with the current, which ﬂows from them to the auxiliary cathode, was changed every 24 h, in accordance with the methodology speciﬁed in DNVGL- RP401 [30]. During the 4-day experiment, the current densities indicated in Table3 were used. The intensity of the current Ii set in the galvanostat was calculated as the product of the required current density ji and area of the sample under the exposure.

Table 3. Density of the current ﬂowing from the samples during the experiment.

2 day one: j1 = 1.5 mA/cm 2 day two: j2 = 0.4 mA/cm 2 day three: j3 = 4.0 mA/cm 2 day four: j4 = 1.5 mA/cm

3.2.5. Method of Determining the Current Capacity of the Tested Alloys The current capacity (also known as the electrochemical efficiency) was determined based on the amount of the sample mass loss due to the outflow of an electric charge of known value. Before measuring the sample masses after the exposure, corrosion by-products were removed from them according to the NACE TM 0190 [32] standard, by immersing the ◦ samples for 10 min in a hot (80 C) cleansing solution made of 20 g chromium trioxide CrO3 3 and 30 mL phosphoric acid H3PO4 dissolved in 1 dm distilled water. The samples were rinsed in tap water and dried in a stream of hot air for 15 min after removing them from the cleansing solution. After cooling down, the samples were weighed to the nearest 0.1 mg. The total electric charge flowing through the sample during the exposure, which was carried out according to the diagram specified in Table3, was calculated from the dependence (1). C = I1·t1 + I2·t2 + I3·t3 + I4·t4 [Ah], (1)

where: Ii—intensity of the polarising current during the next day (A), ti—duration of the current ﬂow of a given intensity (h). The current capacity of alloys was calculated from the Equation (2).

ε = C/∆m [Ah/kg], (2)

where: C—electric charge (Ah), ∆m—mass loss in the tested sample (kg).

4. Results and Discussion 4.1. SEM Microscopic Examination for Alloy Samples Prepared for Tests SEM images of the microstructure of the samples and the maps of indium concentration and its distribution in the scanning area, which were obtained for individual alloys containing a different amount of indium, are presented in Figure4. Each red point corresponds to a pulse of X-ray radiation characteristic to indium. The density of the points, which increases in the subsequent Figure4a–f, corresponds to the increasing concentration of indium in the subsequent samples under tests. It can be seen that the indium is well dissolved in the alloy matrix. It is evenly distributed, there are no separate clusters of this metal in the alloy. Materials 2021, 14, 1755 8 of 17 Materials 2021, 14, x FOR PEER REVIEW 8 of 17

(a)

(b)

(c)

(d)

Figure 4. Cont.

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(e)

(f)

Figure 4.Figure Maps of 4. indiumMaps of distribution indium distribution in the tested in the allo testedy samples alloy with samples different with differentindium content, indium i.e., content, (a) 0.022%; i.e., (a )(b 0.022%;) 0.034%; (b ) 0.034%; (c) 0.048%; (d) 0.088%; (e) 0.133%; (f) 0.197%. (c) 0.048%; (d) 0.088%; (e) 0.133%; (f) 0.197%. (f) A distribution map for zinc atoms in the base alloy (containing 4.24% Zn and 0.022% Figure 4. Maps of indium distributionIn) in isthe presented tested allo iny samples Figure5 with. It is different clearly indium visible content, that zinc i.e., atoms (a) 0.022%; are evenly (b) 0.034%; distributed in the (c) 0.048%; (d) 0.088%; (e) 0.133%; (f) alloy,0.197%. there are no clusters of this element in the alloy matrix.

Figure 5. Zinc distribution maps in the base alloy.

4.2. Electrochemical Studies of Alloys

The determined current capacity values for the tested alloys are presented in Table Figure 5. Zinc distribution maps in the base alloy. 4. The alloy withFigure 0.022% 5. Zinc In content distribution had maps the highest in the base capacity alloy. amounting to approx. 2504 4.2.Ah/kg. Electrochemical The alloy current Studies capacityof Alloys decreases as the indium content increases. With a content of approx. 0.05% In, the capacity was reduced to approx. 2350 Ah/kg. The alloy with The4.2. determined Electrochemical current Studies capacity of Alloys values for the tested alloys are presented in Table an indium content close to 0.2% had a capacity as much as 30% lower than the base alloy 4. The alloy withThe determined0.022% In content current had capacity the highest values capacity for the tested amounting alloys areto approx. presented 2504 in Table4. and it only amounted to approx. 1750 Ah/kg. Table 4 also shows the consumption values Ah/kg. TheThe alloy alloy current with 0.022% capacity In content decreases had theas the highest indium capacity content amounting increases. to approx.With a con- 2504 Ah/kg. for the anode alloy mass, which are reciprocal to the current capacity converted to a yearly tent of approx.The alloy 0.05% current In, the capacity capacity decreases was reduced as the to indium approx. content 2350 Ah/kg. increases. The alloy With with a content of value. This consumption demonstrates the mass loss of the anode material during the an indiumapprox. content 0.05% close In, to the0.2% capacity had a capacity was reduced as much to approx.as 30% lower 2350 than Ah/kg. the Thebase alloyalloy with an year, with the anode’s dissolving current flow having the intensity of 1 A. It can be seen and it onlyindium amounted content to close approx. to 0.2% 1750 had Ah/kg. a capacity Table 4 as also much shows as 30% the lowerconsumption than the values base alloy and that for 0.2% In content the material consumption is as high as 5 kg/(A·year), when com- for the anodeit only alloy amounted mass, which to approx. are reciprocal 1750 Ah/kg. to the Table current4 also capacity shows converted the consumption to a yearly values for pared to 3.5 kg/(A·year) for an alloy containing 0.02% In. value. This consumption demonstrates the mass loss of the anode material during the year, with the anode’s dissolving current flow having the intensity of 1 A. It can be seen that for 0.2% In content the material consumption is as high as 5 kg/(A·year), when com-

pared to 3.5 kg/(A·year) for an alloy containing 0.02% In.

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the anode alloy mass, which are reciprocal to the current capacity converted to a yearly value. This consumption demonstrates the mass loss of the anode material during the year, Materials 2021, 14, x FOR PEER REVIEWwith the anode’s dissolving current ﬂow having the intensity of 1 A. It can be seen10 thatof 17 for

0.2% In content the material consumption is as high as 5 kg/(A·year), when compared to 3.5 kg/(A·year) for an alloy containing 0.02% In. Table 4. Mean current capacity and mean annual mass consumption for the tested Al-Zn-In anode alloys.Table 4. Mean current capacity and mean annual mass consumption for the tested Al-Zn-In anode alloys. Current Capacity Consumption of Anode Material Alloy Number In Content (%) Current Capacity Consumption of Anode Alloy Number In Content (%) (Ah/kg) (kg/(A∙Year)) · 1 0.022 2503.98(Ah/kg) Material 3.48 (kg/(A Year)) 2 10.034 0.022 2381.22 2503.98 3.54 3.48 3 20.048 0.034 2347.54 2381.22 3.75 3.54 3 0.048 2347.54 3.75 4 40.088 0.088 2290.13 2290.13 3.82 3.82 5 50.133 0.133 2247.13 2247.13 3.88 3.88 6 60.197 0.197 1749.35 1749.35 5.03 5.03

In FigureIn Figure 6, 6illustrating, illustrating the the changes changes in inthe the current current capacity capacity of ofthe the tested tested alloys, alloys, a sharp a sharp capacitycapacity drop drop is visible is visible when when the the indium indium cont contentent reaches reaches 0.2%. 0.2%. In the In the range range of 0.05–0.15% of 0.05–0.15% In,In, the the capacity capacity drop drop is moderate. is moderate. It can It can be bestated stated that that the the alloys alloys with with the the maximum maximum 0.05% 0.05% In Incontent content are are practically practically applicable applicable for for cathodic cathodic protection protection in seawater. in seawater. For For comparison: comparison: in inwork work [21] [21 the] the optimum optimum concentration concentration 0.02% 0.02% In Inwas was found found when when zinc zinc content content was was 5%, 5%, andand in work in work [29] [29 0.015%] 0.015% In Inand and 3.5% 3.5% Zn, Zn, respectively. respectively.

3000

2500

2000 (Ah/kg) ε

1500

1000 0 0.05 0.1 0.15 0.2 0.25 In content (% wt.)

Figure 6. Diagram for the mean current capacity of Al-Zn-In alloys depending on the In content. Figure 6. Diagram for the mean current capacity of Al-Zn-In alloys depending on the In content. Values of the potentials of the tested samples are presented in Table5. In this table, theValues OCP of potential the potentials values of (before the tested starting samples the are polarisation) presented in and Table the 5. CCPIn this potentials table, the of OCPthe potential polarised values samples (before at the starting end of the each polarisation) experiment and phase, the CCP as wellpotentials as the of density the polar- of the isedpolarising samples currentat the end flowing of each out experiment from the samples phase, inas eachwell phaseas the (day),density are of presented. the polarising As the currentindium flowing content out increases from the from samples 0.02 toin 0.05%,each phase the OCP (day), potential are presented. value is moreAs the negative indium by contentabout increases 50 mV, which from 0.02 is advantageous to 0.05%, the OCP for using potential this alloyvalue as is amore sacrificial negative anode. by about A further 50 mV,increase which inis advantageous the indium content for using no longerthis alloy changed as a sacrificial the OCP anode. potential A further of the alloy.increase During in thepolarisation indium content of alloys, no longer a particular changed dependencethe OCP potential can be of clearlythe alloy. seen, During namely polarisation the higher of alloys,density a ofparticular the current dependence flowing can out be from clea therly sample,seen, namely the more the high positiveer density value of of the the cur- CCP rentpotential. flowing out With from a very the sample, high current the more load positive for the value anode of on the the CCP third potential. day of With exposure, a very the highpotential current ofload the for polarised the anode samples on the was third more day positiveof exposure, than the the potential OCP potential, of the polarised by as much samplesas 200 was mV. more positive than the OCP potential, by as much as 200 mV. According to the adopted experiment procedure in accordance with DNVGL-RP- B401 [30], the main parameter considered when it comes to using the tested alloy as a sacrificial anode for offshore structure cathodic protection is the sample’s potential at the end of the exposure (after 96 h of polarisation). Figure 7 shows the OCP and CCP potential values at the end of the exposure. The alloy with 0.048% In content had the most negative

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Table 5. Mean open circuit potential (OCP) and mean closed circuit potentials (CCP) of the tested alloys vs. Materials 2021, 14, x FOR PEER REVIEW 11 of 17 Ag/AgCl/seawater electrode after the next exposure stages.

CCP (mV) OCP Alloy Number In Content (%) CCP value. The(mV) alloys with 0.05–0.20%After 24 h indiumAfter content 48 h have similarAfter 72 CCP h potentialAfter values 96 h 2 2 2 2 and they are noticeably morej = 1.5 negative mA/cm (by 30–40j = 0.4 mV) mA/cm than alloysj = 4.0 with mA/cm 0.02–0.04%j = 1.5 content. mA/cm 1 0.022 −1065 −956 −935 −881 −919 Table 5.2 Mean open circuit 0.034 potential (OCP)− 1079and mean closed −circuit968 potentials (CCP)−908 of the tested alloys−895 vs. Ag/AgCl/sea-−922 water electrode3 after the 0.048 next exposure stages.−1099 −988 −919 −968 −970 4 0.088 −1100 −1045 −963 −930 −963 5 0.133 −1108 −1013 −938CCP (mV) −942 −945 OCP Alloy Number6 In Content 0.197 (%) −1099 After− 24986 h After −48969 h After −72912 h After −96937 h (mV) j = 1.5 mA/cm2 j = 0.4 mA/cm2 j = 4.0 mA/cm2 j = 1.5 mA/cm2 1 0.022 According−1065 to the adopted−956 experiment−935 procedure in− accordance881 with− DNVGL-RP-919 2 0.034 B401 [30−1079], the main parameter−968 considered−908 when it comes− to895 using the tested−922 alloy as a 3 0.048 sacriﬁcial−1099 anode for offshore−988 structure cathodic−919 protection is−968 the sample’s potential−970 at the 4 0.088 end of the−1100 exposure (after−1045 96 h of polarisation).−963 Figure7 shows−930 the OCP and CCP−963 potential 5 0.133 values at−1108 the end of the exposure.−1013 The alloy−938 with 0.048% In− content942 had the most−945 negative CCP value. The alloys with 0.05–0.20% indium content have similar CCP potential values 6 0.197 −1099 −986 −969 −912 −937 and they are noticeably more negative (by 30–40 mV) than alloys with 0.02–0.04% content.

CCP after 96 h OCP

-800 -850 -900 -950 (mV) -1000

Ag/AgCl -1050 E -1100 -1150 -1200 0 0.05 0.1 0.15 0.2 0.25 In content (% wt.)

Figure 7. Diagram for the mean OCP and mean CCP potentials for the tested Al-Zn-In alloys in Figurerelation 7. Diagram to the In for content. the mean OCP and mean CCP potentials for the tested Al-Zn-In alloys in relation to the In content. 4.3. Post-Exposure Surface of the Samples 4.3. Post-ExposureThe pictures Surface in Figure of the8 Samplesillustrate the surface view of the tested anode alloys after the exposure,The pictures at 10 in× Figureand 60 ×8 illustratemagniﬁcation. the surface view of the tested anode alloys after the exposure,Figure at 10×8 demonstrates and 60× magnification. that corrosion due to dissolution of the alloy by the polarising currentFigure was 8 demonstrates general in nature that co andrrosion took due place to over dissolution the entire of surfacethe alloy of by the the samples. polarising How- currentever, was the surfacegeneral morphologyin nature and of took individual place over samples the entire is diversiﬁed. surface of As the the samples. indium How- content ever,increases, the surface a generally morphology uniform of individual corrosion samp patternles is changed diversified. more As and the more indium to ancontent uneven increases,one. The a sizegenerally of local uniform metal cavitiescorrosion (pits) pattern increased, changed and more they becameand more longer to an and uneven deeper. one.Local The cavitiessize of local on the metal sample cavities surface (pits) were increased, the smallest and inthey the became case of thelonger sample and containingdeeper. Local0.022% cavities In, andon the the sample largest surface cavities were were the observed smallest onin the case surface of the of thesample samples containing with the 0.022%0.133% In, andand 0.197%the largest In content. cavities were observed on the surface of the samples with the 0.133% and 0.197% In content. Figure 9 shows the 3D surface images for the samples after the exposure and the surface height profiles obtained along the X-axis in the central part of the 3D views. Based on the diagrams of the height profiles, the surface roughness coefficient Ra was calculated for each sample, according to the formula (3). The roughness coefficient values for individual samples are presented in Table 6.

Ra = (1/n)·Σyi (3)

Materials 2021, 14, x FOR PEER REVIEW 12 of 17

Materials 2021, 14, 1755 where: yi—deviation of the ordinate value of the ith measurement point from the 12mean of 17 ordinate value of all the points that make up the profile plot, n—number of measurement points.

magn. 10×: magn. 60×: magn. 10×: magn. 60×:

0.022% In 0.034% In

0.048% In 0.088% In

0.133% In 0.197% In

Figure 8. Post-exposure surface views of the tested alloys.

Figure9 shows the 3D surface images for the samples after the exposure and the The yi are absolute values of differences between the ordinates that make up the pro- surface height profiles obtained along the X-axis in the central part of the 3D views. file plot and the mean value of all ordinates. The Ra value reflects the size of the cavities (pits)Based on on the the surface diagrams of the of tested the height samples. profiles, So, a the value surface of the roughness roughness coefficient coefficient R calcu-a was latedcalculated according for each to the sample, Formula according (3) can be to theconsid formulaered as (3). a measure The roughness of the size coefficient of the defects values for individual samples are presented in Table6. on the surface of the samples. The higher the Ra value, the greater the pitting depths on the sample surface, and therefore the greater the weight loss of the tested alloy—and Ra = (1/n)·Σyi (3) shorter operation time of sacrificial anodes and the lower their current capacity.

where: yi—deviation of the ordinate value of the ith measurement point from the mean Tableordinate 6. Surface value roughness of all the coefficient points that of the make tested up anode the proﬁlealloy samples plot, n—numberafter the exposure. of measure-

ment points.Alloy Number In Content (%) Roughness Coefficient Ra 1 0.022 35.25 μm Table 6. Surface roughness coefﬁcient of the tested anode alloy samples after the exposure. 2 0.034 44.92 μm

Alloy Number3 In Content0.048 (%) Roughness62.99 Coefﬁcient μm Ra 41 0.088 0.022 72.34 35.25 μµm 52 0.133 0.034 91.78 44.92 μµm 63 0.197 0.048 94.62 62.99 μµm 4 0.088 72.34 µm 5 0.133 91.78 µm 6 0.197 94.62 µm

Materials 2021, 14, x FOR PEER REVIEW 13 of 17

6 0.197 94.62 μm

The obtained roughness coefficient values confirm the visual assessment described on the basis of Figure 8, which pertains to the increasing unevenness and depth of corrosion defects on the sample surfaces with an increasing In content. A higher corrosion rate (greater weight loss) may result from the fact that a passive oxide layer is more easily Materials 2021, 14, 1755 formed on the surface of alloys with higher indium content, which then breaks and13 oflocal 17 dissolution of Al-Zn-In alloy occurs [33]. The increased mass loss induces the reduced current capacity of the alloy.

0.034% In: 0.22% In:

0.048% In: 0.088% In:

Materials 2021, 14, x FOR PEER REVIEW 14 of 17

0.133% In: 0.197% In:

FigureFigure 9.9.3D 3D viewview andand surfacesurface proﬁlesprofiles of of the the tested tested alloys alloys after after the the exposure. exposure.

5. Discussion The electrochemical properties of the individual components of the Al-Zn-In alloy differ significantly. According to the voltage series of metals (giving the potential values versus the standard hydrogen electrode), the lowest potential is revealed by aluminum −1.66 V, then zinc −0.76 V and indium −0.33 V. Therefore, corrosive galvanic cells are formed on the surface of the alloy in contact with the electrolyte, that arise between micro- regions containing metals or intermetallic alloys with different potentials. In these cells, Al is the dissolving anode, and Zn and In are the cathode. The anode-cathode potential difference in Al-In microcells is very large, so the intensity of the corrosion current is also high. This means a high corrosion rate in these microcells. Figure 10 shows the result of the SEM test for indium content at various locations in the alloy. The tests were carried out on the sample No. 6 with a content 0.197% In in the whole alloy. The measurements were taken at 50 points along the section marked in yellow in Figure 10a. The graph of indium content along this section is shown in the red line in Figure 10b,d,f.

(a) (b)

(c)

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The yi are absolute values of differences between the ordinates that make up the profile plot and the mean value of all ordinates. The Ra value reflects the size of the cavities (pits) on the surface of the tested samples. So, a value of the roughness coefficient calculated according to the Formula (3) can be considered as a measure of the size of the defects on the surface of the samples. The higher the Ra value, the greater the pitting depths on the sample surface, and therefore the greater the weight loss of the tested alloy—and shorter operation time of sacrificial anodes and the lower their current capacity. The obtained roughness coefficient values confirm the visual assessment described on the basis of Figure8, which pertains to the increasing unevenness and depth of corrosion defects on the sample surfaces with an increasing In content. A higher corrosion rate (greater weight loss) may result from the fact that a passive oxide layer is more easily formed on the surface of alloys with higher indium content, which then breaks and local dissolution of Al-Zn-In alloy occurs [33]. The increased mass loss induces the reduced current capacity of the alloy.

5. Discussion The electrochemical properties of the individual components of the Al-Zn-In alloy differ signiﬁcantly. According to the voltage series of metals (giving the potential values versus the standard hydrogen electrode), the lowest potential is revealed by aluminum −1.66 V, then zinc −0.76 V and indium −0.33 V. Therefore, corrosive galvanic cells are formed on the surface of the alloy in contact with the electrolyte, that arise between micro- regions containing metals or intermetallic alloys with different potentials. In these cells, Al is the dissolving anode, and Zn and In are the cathode. The anode-cathode potential difference in Al-In microcells is very large, so the intensity of the corrosion current is also high. This means a high corrosion rate in these microcells. Figure 10 shows the result of the SEM test for indium content at various locations in the alloy. The tests were carried out on the sample No. 6 with a content 0.197% In in the whole alloy. The measurements were taken at 50 points along the section marked in yellow in Figure 10a. The graph of indium content along this section is shown in the red line in Figure 10b,d,f. There is a quantitative differentiation of the In content in particular places on the tested surface of the sample. Its amount at the point P1 was about 0% (Figure 10c), while at the point P2 it was as much as 2.27% (Figure 10e). This means that after the alloy is immersed in an electrolyte, galvanic microcells will be formed near the point P2 and a greater weight loss of the alloy will occur compared to places with lower indium content. When comparing alloys with different indium content, the increasing amount of this element contributes to the increased activity of corrosion microcells in the alloy. Thus, the corrosion susceptibility of the alloy increases with increasing indium content. Additionally, electrolyte alkalisation takes place around the cathodes in the corrosion cells (OH− ions generated as a result of the oxygen reduction reaction at the cathode). Such an electrolyte may accelerate the corrosion of the alloy matrix, as aluminum is not resistant to the alkaline environment. In the research presented in this work, the intensity of the corrosion processes described above in the microcells was enhanced by the current polarizing anodically the samples during exposure, which also causes the dissolution of the alloy. Therefore, along with the increase in indium content, the corrosion losses (pitting on the surface of the samples) were greater and greater and the weight loss of the samples increased. Figure6 shows that a rapid increase in the intensity of the corrosion processes of the working sacriﬁcial anode occurs when the amount of indium in the Al-Zn-In alloy is above 0.15%. The effect of this is a reduction in the current capacity of such an alloy. Materials 2021, 14, x FOR PEER REVIEW 14 of 17

Figure 9. 3D view and surface profiles of the tested alloys after the exposure.

5. Discussion The electrochemical properties of the individual components of the Al-Zn-In alloy differ significantly. According to the voltage series of metals (giving the potential values versus the standard hydrogen electrode), the lowest potential is revealed by aluminum −1.66 V, then zinc −0.76 V and indium −0.33 V. Therefore, corrosive galvanic cells are formed on the surface of the alloy in contact with the electrolyte, that arise between micro- regions containing metals or intermetallic alloys with different potentials. In these cells, Al is the dissolving anode, and Zn and In are the cathode. The anode-cathode potential difference in Al-In microcells is very large, so the intensity of the corrosion current is also high. This means a high corrosion rate in these microcells. Figure 10 shows the result of the SEM test for indium content at various locations in the alloy. The tests were carried out on the sample No. 6 with a content 0.197% In in the Materials 2021, 14, 1755 whole alloy. The measurements were taken at 50 points along the section marked in15 yel- of 17 low in Figure 10a. The graph of indium content along this section is shown in the red line in Figure 10b,d,f.

(a) (b)

Materials 2021, 14, x FOR PEER REVIEW 15 of 17

(e) (f)

FigureFigure 10. 10.SEM SEM tests tests ofof thethe locallocal contentcontent ofof InIn inin thethe alloy: ( a) measurement section, section, ( (bb)) changes changes in in the the amount amount of of In In along along thethe measurement measurement section section (red(red line),line), ((cc)) locallocal elementelement content of the alloy at at the the point point P1, P1, ( (dd)) graph graph of of the the indium indium content content along the measurement section with the location of the point P1, (e) local element content of the alloy at the point P2, (f) along the measurement section with the location of the point P1, (e) local element content of the alloy at the point P2, graph of the indium content along the measurement section with the location of the point P2. (f) graph of the indium content along the measurement section with the location of the point P2. There is a quantitative differentiation of the In content in particular places on the 6. Conclusions tested surface of the sample. Its amount at the point P1 was about 0% (Figure 10c), while at theThe point obtained P2 it was test as results much demonstrateas 2.27% (Figure that 10e). Al-Zn-In This means alloys withthat after an In the content alloy is of 0.05–0.15%immersed incan an be electrolyte, successfully galvanic used as microcel sacrificialls anodeswill be informed these near applications the point and P2 cases, and a in whichgreater the weight desired loss anode of the potentialalloy will value occur shouldcompared be lowerto places by severalwith lower dozen indium mV. However,content. the serviceWhen comparing life of such alloys anodes with is reduceddifferent byindium 5–10% content, (operating the increasing time until amount the complete of this dissolutionelement contributes of the anode), to the whenincreased compared activity to of anodes corrosion with microcells 0.02% In in content. the alloy. Thus, the corrosionIncreasing susceptibility the In content of the above alloy increases 0.15% has with a very increasing negative indium effect on content. the current capacity of Al-Zn-InAdditionally, alloy, which electrolyte is significantly alkalisation decreased takes place in around such a case.the cathodes For 0.2% in Inthe content, corrosion the currentcells (OH capacity− ions generated is lower by as asa result much of as the 30%, oxygen in comparison reduction toreaction the alloy at the with cathode). 0.02% In, Such and thean electrolyte consumption may of accelerate material the for thiscorrosion sacrificial of the anode alloy increasesmatrix, as dramaticallyaluminum is not and resistant amounts toto asthe much alkaline as approx. environment. 5 kg/(A ·year), in comparison to 3.5 kg for 0.02% In content. WithIn the consideration research presented of all thein this determined work, the electrochemical intensity of the parameterscorrosion processes of the tested de- alloysscribed (i.e., above the potentialin the microcells of the operating was enhanced anode, by its currentthe current capacity polarizing and consumption anodically the rate forsamples the material), during exposure, the economic which aspects also causes (i.e., th anodee dissolution service timeof the and alloy. the Therefore, requirement along for itswith replacement the increase with in aindium new one), content, as well the as co therrosion environmental losses (pitting aspects on (i.e.,the surface contamination of the withsamples) the corrosionwere greater by-products and greater after and dissolution the weight loss of the of anode),the samples it can increased. be stated that the Al-Zn-InFigure alloys 6 shows with that no more a rapid than increase 0.05% Inin the content intensity are practically of the corrosion applicable processes for cathodic of the protectionworking sacrificial in seawater. anode occurs when the amount of indium in the Al-Zn-In alloy is above 0.15%. The effect of this is a reduction in the current capacity of such an alloy.

6. Conclusions The obtained test results demonstrate that Al-Zn-In alloys with an In content of 0.05– 0.15% can be successfully used as sacrificial anodes in these applications and cases, in which the desired anode potential value should be lower by several dozen mV. However, the service life of such anodes is reduced by 5–10% (operating time until the complete dissolution of the anode), when compared to anodes with 0.02% In content. Increasing the In content above 0.15% has a very negative effect on the current capacity of Al-Zn-In alloy, which is significantly decreased in such a case. For 0.2% In content, the current capacity is lower by as much as 30%, in comparison to the alloy with 0.02% In, and the consumption of material for this sacrificial anode increases dramatically and amounts to as much as approx. 5 kg/(A·year), in comparison to 3.5 kg for 0.02% In content. With consideration of all the determined electrochemical parameters of the tested alloys (i.e., the potential of the operating anode, its current capacity and consumption rate for the material), the economic aspects (i.e., anode service time and the requirement for its replacement with a new one), as well as the environmental aspects (i.e., contamination with the corrosion by-products after dissolution of the anode), it can be stated that the Al- Zn-In alloys with no more than 0.05% In content are practically applicable for cathodic protection in seawater.

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Author Contributions: Conceptualisation, methodology, formal analysis and writing—original draft preparation, K.Z.; investigation, K.Z., J.O., M.C., P.I. and K.D. (Kinga Domanska); supervision, validation, K.D. (Kazimierz Darowicki). All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing is not applicable to this article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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