Corrosion Protection of Injection Molded Porous 440C Stainless Steel by Electroplated Zinc Coating

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Article Corrosion Protection of Injection Molded Porous 440C Stainless Steel by Electroplated Zinc Coating

Matti Kultamaa 1, Kari Mönkkönen 2, Jarkko J. Saarinen 1,* and Mika Suvanto 1,*

1 Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland; matti.kultamaa@uef.fi 2 Karelia University of Applied Sciences, FI-80200 Joensuu, Finland; kari.monkkonen@karelia.fi * Correspondence: jarkko.j.saarinen@uef.fi (J.J.S.); mika.suvanto@uef.fi (M.S.)

Abstract: Zinc electroplating was used to enhance corrosion resistance of porous metal injection molded 440C stainless steel. Controlled porosity was achieved by the powder space holder technique and by using sodium chloride as a space holder material. The internal pore structure of porous 440C was deposited by zinc using electroplating with three different electrolytes of zinc acetate, zinc sulfate, and zinc chloride. Our results show that all zinc depositions on porous 440C samples signiﬁcantly improved corrosion resistance. The lowest corrosion was observed with zinc acetate at 30 wt.% porosity. The developed zinc coated porous 440C samples have potential in applications in corrosive environments.

Keywords: corrosion protection; metal injection molding; stainless steel; 440C; porous metal; zinc electroplating

Citation: Kultamaa, M.; Mönkkönen, K.; Saarinen, J.J.; Suvanto, M. Corrosion Protection of Injection 1. Introduction Molded Porous 440C Stainless Steel Stainless steels are widely used in various applications, ranging from simple everyday by Electroplated Zinc Coating. items such as cutlery and kitchen equipment to highly complicated products in automotive Coatings 2021, 11, 949. https:// and medical industries. The wide use of these materials is based on their suitable properties: doi.org/10.3390/coatings11080949 stainless steel materials are generally mechanically strong, hard, and corrosion resistant [1,2]. Martensitic 440C is the strongest of the martensitic stainless-steel grades. This is Academic Editor: Armando due to the high carbon content (nominal composition 1.10 wt.%), along with the carbides Yáñez-Casal present in the crystal structure [3,4]. As typical martensitic stainless-steel grades, 440C can also be made harder by heat treating, for example, by annealing, quenching, or tempering Received: 20 May 2021 unlike stainless steel grades from other crystal structure families [3,5,6]. Martensitic 440C Accepted: 5 August 2021 Published: 9 August 2021 is typically used as bearings, knives, medical equipment, and in automotive industry due to its hardness and wear resistance [2,3,5–7]. However, the high carbon content comes

Publisher’s Note: MDPI stays neutral with a drawback: The corrosion resistance of type 440C stainless steel is only average with regard to jurisdictional claims in due to the carbide precipitation in the crystal structure despite a high chromium content published maps and institutional afﬁl- (17.00 wt.%) [4,7,8]. Hence, the corrosion resistance of 440C is among the weakest of iations. stainless-steel grades [8]. The most typically used stainless steel grades are martensitic, ferritic, and austenitic stainless steel, and the corrosion resistance differences between these grades originate from their chemical composition [1]. For example, ferritic 444 stainless steel has a higher stress corrosion cracking resistance compared to 440C, whereas austenitic 316L containing Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. molybdenum has good corrosion resistance in chloride rich environments [9,10]. This article is an open access article Martensitic 440C is especially susceptible to pitting corrosion similar to other stainless- distributed under the terms and steel grades [11,12]. In addition, crevice and intergranular corrosions as secondary corro- conditions of the Creative Commons sion mechanisms were reported for martensitic stainless steels [13,14]. However, several Attribution (CC BY) license (https:// methods can be used to improve stainless steel corrosion resistance. For instance, the creativecommons.org/licenses/by/ corrosion resistance of austenitic 316L stainless steel was improved with a Nb-coating 4.0/). by physical vapor deposition (PVD) [15] or by using anticorrosive polypyrrole ﬁlms by

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electrodeposition [16]. Common alternatives such as galvanization or zinc-plating are also widely used to protect iron and steel from corrosion, which can also be applied on stainless steel [17]. Two common galvanization methods are hot-dip galvanization, which involves immersing the steel part in molten zinc [18], and electrogalvanization, which utilizes electrolysis to coat the sample material with zinc [19]. Zinc protects steel and iron both physically as a coating and electrochemically via cathodic protection [20]. Cathodic protection is particularly important; whenever the zinc coating on the steel surface becomes damaged [21], the less noble zinc will act as a sacrificial anode and corrode instead of iron in the formed galvanic cell [18]. Zinc plating is a technique that has been used for corrosion protection of both steel and iron for decades. It has also been studied for protecting stainless steel. For example, electrolytic baths containing ZnSO4 have been used to electrochemically coat 316L and 316 stainless steel grades [17,22]. Metal injection molding (MIM) is a manufacturing method that combines traditional powder metallurgy with plastic injection molding techniques. MIM can produce metal parts with high dimensional accuracy and intricate geometries. Porous metals with micro- sized pores can also be fabricated by combining MIM with powder space holder (PSH) technique. In PSH-MIM, a spacer material such as NaCl, carbamide, or calcium carbonate is used to create porosity that is achieved by removing the spacer material after IM process. Much of PSH research has focused on the production of porous titanium for the medical industry, especially in biomedical applications as an implant [23,24]. MIM has gained a solid commercial position as a manufacturing technique for stainless steel parts [24]. The fabrication of porous 316L stainless steel by combining the MIM and PSH methods has been demonstrated by using poly(methyl methacrylate) (PMMA) as a space holder [25–27]. In this study martensitic 440C stainless steel is used as a feedstock in MIM, although the extensive shrinkage and formation of carbide networks during sintering limit its commercial potential [28,29]. As far as the authors know, the corrosion protection of porous stainless-steel parts has not been widely reported. Sintered stainless steel parts, in general, possess lower corrosion resistance compared to wrought or cast parts. This is due to the inherent porosity of the sintered parts [30]. Thus, corrosion protection is of great importance, especially in the case of porous parts by PSH method, where porosity can be significantly higher than in the traditional injection molded parts. Open porosity can result in internal corrosion of the sintered structure due to a higher surface area susceptible to the electrolyte. Crevice corrosion can also become a problem if water becomes trapped inside the small pores of porous stainless steel. This is particularly detrimental in environments with chloride [30–32]. A recent study presented a method in order to improve corrosion resistance of 316L grade with porosity of 11.2–13.0% by the electrodeposition of polypyrrole (PPy) on the steel surface [33]. MIM manufactured 440C stainless steel structures have been widely used in applications in which hardness, high strength, and resistance to wear are required [34]. The use of sodium chloride as a space holder material to produce porous or foam metals is also known [35]. However, as far as the authors know, the internal pore structure of such porous stainless steel produced by PSH-MIM has not been utilized in corrosion protection. This study focuses on the deposition of corrosion protective zinc particles into the internal pore structure of injection molded porous 440C via electrodeposition. Zinc in the pores acts as a sacrificial anode that extends the usability of the porous 440C steel in corrosive environments with electrodeposition, providing a scalable and cost-efficient surface functionalization. The aim of this study is to examine the pitting corrosion resistance potential of zinc coating by electroplating on porous MIM 440C samples. Zinc coatings were produced by a simple zinc electrolysis bath containing either zinc acetate, zinc sulfate, or zinc chloride. Samples with three controlled porosities (10, 20, and 30 wt.%) were fabricated by adding sodium chloride space holder material. Coatings 2021, 11, 949 3 of 11

2. Materials and Methods 2.1. Metal Injection Molding (MIM) Martensitic polyMIM 440C stainless steel (polyMIM GmbH, Bad Sobernheim, Ger- many) was used as feedstock in the metal injection molding (MIM) process. The typical composition of the used 440C is shown in Table1. Sodium chloride was used as space holder material. Both 440C and NaCl were milled by using the Ultra Centrifugal Mill ZM 200 (Retsch GmbH, Haan, Germany). After milling, the size of NaCl particles was determined with a Vibratory sieve shaker AS 200 digit (Retsch GmbH, Haan, Germany). A size distribution of 200–315 µm was chosen for the experiments.

Table 1. Typical composition of PolyMIM 440C stainless steel (as sintered in % by weight).

Composition Fe C Ni Cr Mo Mn Si Nb S P > - 0.85 - 16.0 - - - 1.0 - - < Balance 1.0 0.6 18.0 0.75 1.0 1.0 2.0 0.03 0.04

Milled 440C feedstock and sieved NaCl were mixed, and paraffin (VWR Chemicals BDH, Leuven, Belgium) was added into the mixture to reduce viscosity for the injection molding phase. Three types of mixtures were prepared based on the weight percentage of NaCl in the mixture: 10, 20 and 30 wt.%. The percentage of paraffin was set to 1.0–2.5 wt.%. The HAAKETM MiniJet II injection molding system (Thermo Fisher Scientific, Karl- sruhe, Germany) was used to compact the feedstock mixtures into the desired shapes. For cylindrical, coin-shaped samples, the injection pressure of 450 bar (350 bar for samples including 10 wt.% NaCl) and injection time (holding pressure) of 5 s were used. The samples were solvent debinded after injection molding, in which the binder incorporated in the 440C feedstock and NaCl was removed from the samples. Debinding was carried out in a distilled water bath at 60 ◦C for a minimum of 15 h. Samples with a higher weight percentage of NaCl required a longer debinding time. Table2 shows the average masses of the samples before and after debinding with standard deviations. The mass loss was induced by the removal of NaCl and the binder material from the samples during debinding.

Table 2. Porous 440C sample masses before and after debinding.

NaCl (wt.%) Before Debinding (g) After Debinding (g) Mass Loss (%) 10 12.62 ± 0.08 10.90 ± 0.08 13.6 ± 0.6 20 11.33 ± 0.09 8.72 ± 0.09 23.0 ± 0.5 30 10.18 ± 0.10 6.82 ± 0.13 33.0 ± 0.7

Before sintering, a hole with a diameter of 3 mm was drilled to the coin-shaped samples that allowed the sample attachment into the corrosion test chamber for accurate corrosion testing. The samples were sintered using a Carbolite GERO, model HTK 8 MO/16 furnace (Carbolite Gero Ltd., Neuhausen, Germany). The sintering was carried out in nitrogen atmosphere with the following cycle: from room temperature to 600 ◦C, then held at 1 h at this temperature and followed by an increase from 600 ◦C to 1290 ◦C with 2 h holding time. Finally, the furnace was cooled down from 1290 ◦C to 80 ◦C. Coatings 2021, 11, 949 4 of 11

2.2. Zinc Coating by Electroplating Sintered 440C samples were electroplated with zinc 48 h after the sintering. Three different types of electrolytes were prepared with zinc-containing compounds dissolved in distilled water: zinc acetate (anhydrous, 99.98%, pH = 5, Alfa Aesar, Ward Hill, MA, USA, pH = 8), zinc sulfate heptahydrate (≥99.0%, Honeywell, Seelze, Germany), and zinc chloride (≥98%, pH = 4, Sigma-Aldrich, Steinheim, Germany). The electrolytes were prepared by dissolving the zinc containing compounds in 50 mL of distilled water with electrolyte concentration set to 0.2 M. It is noteworthy that no additives were added to the electrolytes. A simple electrolytic bath consisted of the electrolyte in a 250 mL glass beaker, a mag- netic stirrer, power supply, and a 3 mm thick zinc foil (25 × 25 mm2, >99.99%, Goodfellow, Huntingdon, UK) as a zinc donating anode. Electroplated samples were attached to the system as a cathode. One anode system was selected here for simplicity. Electrolysis for each sample was carried out for 1 h with a constant voltage of 2.0 V. The electric current varied based on the used electrolyte from around 0.04 A (corresponding to a current den- sity of 53.3 mA/cm2) for zinc acetate, 0.05 A (66.7 mA/cm2) for zinc sulfate, and 0.08 A (106.7 mA/cm2) for zinc chloride. The electroplated samples were washed with distilled water immediately after the electrolysis. Distilled water was removed from the samples by using a pressurized air gun followed by drying in a fume hood.

2.3. Corrosion Testing Corrosion testing was carried out in a neutral (pH 6.5–7.2) 5% NaCl salt spray chamber (VLM CCT1000S, VLM GmbH, Bielefeld, Germany) at room temperature for the duration of 240 h following the European standard protocol (BS EN 1670:2007, Building hardware— Corrosion resistance—Requirements and test methods). After the corrosion test, each sample was rinsed with distilled water.

2.4. Characterization Porous sintered samples were analyzed by using a ﬁeld emission scanning electron mi- croscope (FESEM, Hitachi S-4800, Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS). Investigation of the internal pore structures of the porous stainless-steel samples was carried out from cross-sectional cuts of the samples.

3. Results and Discussion 3.1. Morphology of Porous Structures The pore morphology of the MIM fabricated porous stainless-steel structure was induced by the size and shape of the used space holder material crystals (sodium chloride). The spaces occupied by the NaCl crystals during the injection molding phase were con- verted into pores during the debinding phase once NaCl was removed from the samples. Figure1 shows the SEM images of the sintered samples with different porosities of 10, 20, and 30 wt.%. The sintered, coin-shaped samples were broken in half mechanically, and the internal pore structure at the cross-sectional fracture plane was investigated and imaged with SEM. Increase in the NaCl content clearly increases the porosity of the sintered samples. The sample with the highest porosity (Figure1c) has a highly porous structure with interconnected pores that extend deep into the structure. By comparison, the cross- sectional fracture surfaces of the samples with lower porosities were more closed and the number of small pores in the sample was lower (Figure1a,b). Coatings 2021, 11, 949 5 of 11 Coatings 2021, 11, x FOR PEER REVIEW 5 of 11

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Figure 1.1. LowLow magniﬁcationmagnification SEM SEM images images of theof the internal internal pore pore structure structure of sintered of sintered 440C stainless440C stainless steel samples. steel samples. The amount The Figureofamount NaCl 1. isof Low 10NaCl wt.% magnification is (10a), wt% 20 wt.% (a), SEM20 (b ),wt% andimages (b 30), wt.%and of the30 (c ). wt%internal (c). pore structure of sintered 440C stainless steel samples. The amount of NaCl is 10 wt% (a), 20 wt% (b), and 30 wt% (c). 3.2. Morphology of the Electroplated Zinc Coating 3.2.3.2. Morphology MorphologyThree different of of the the zinc Electroplated Electroplated electrolytes Zinc Zinc were Coating used for electroplating from which zinc acetate was observedThreeThree different different to possess zinc betterelectrolytes corrosion were resistan used for ce electroplating than zinc sulfate from or which zinc chloride zinc acetate sam- wasples. observed observedTherefore, to to zincpossess possess acetate better better was corrosion used corrosion for resistan elec resistancetrolyticce than zinc than zinc deposition zinc sulfate sulfate or for zinc orall chloride zincdifferent chloride sam- po- ples.rositiessamples. Therefore, (10, Therefore, 20, and zinc 30 zincacetate wt.% acetate NaCl),was wasused whereas used for elec for zinctrolytic electrolytic chloride zinc and zincdeposition zinc deposition sulfate for allwere for different all used different with po- rositiestheporosities most (10, porous (10, 20, 20,and sample and 30 wt.% 30 (30 wt.% NaCl),wt.% NaCl), NaCl). whereas whereas zinc zincchloride chloride and zinc and sulfate zinc sulfate were wereused usedwith thewith mostFigure the mostporous 2 shows porous sample SEM sample (30 images wt.% (30 wt.%NaCl).of the NaCl). internal pore structure of samples with different porositiesFigureFigure after 22 showsshows electrochemical SEMSEM imagesimages zinc ofof thethedeposition internalinternal using porepore structurestructurezinc acetate. ofof samplessamples No zinc with withwas differentobserveddifferent porositiesinsideporosities the aftersample electrochemical with the lowest zinc porosity deposition ((10 using wt.% NaCl)zinc acetate. in Figure No 2a).zinc This was was observed prob- insideablyinside due the the to sample sample a limited with with open the the porosity lowest lowest porosity porosityon the samp ((10 ((10 lewt.% wt.% surface. NaCl) NaCl) No in in significant Figure Figure 22a).a). differences This was prob- were ablyobservedably due due to in to a the alimited limited deposited open open zincporosity porosity amount on onthe either thesamp in samplesidele surface. 20 surface. wt.% No (Figure significant No signiﬁcant 2b) or differences 30 wt.% differences (Figure were observed2c)were samples. observed in the However, indeposited the deposited there zinc was amount zinc a clear amount either difference in eitherside 20in inside thewt.% deposited 20 (Figure wt.% (Figure2b) zinc or morphology 302b) wt.% or 30 (Figure wt.% be- 2c)tween(Figure samples. the2c) samples: samples. However, the However, 20there wt.% was there sample a clear was had adifference clear zinc differencedeposits in the in deposited in more the depositedagglomerated zinc morphology zinc form morphol- with be- tweensmallogy between clusters,the samples: the whereas samples: the 20 the wt.% the30 wt.% sample 20 wt.% sample had sample zi concntained deposits had zinc a more in deposits more web-like agglomerated in more pattern agglomerated withform elon-with form with small clusters, whereas the 30 wt.% sample contained a more web-like pattern smallgated clusters,shapes. whereas the 30 wt.% sample contained a more web-like pattern with elon- with elongated shapes. gated shapes.

Figure 2. SEM images of the internal pore structure of sintered 440C stainless steel samples after zinc acetate electrodeposition. The amount of NaCl is 10 wt.% (a), 20 wt.% (b), and 30 wt.% (c). Figure 2. SEMSEM images of the internal pore structure ofof sinteredsintered 440C440C stainlessstainless steelsteel samplessamples afterafter zinczinc acetate acetate electrodeposi- electrodepo- sition.tion. The The amount amount of of NaCl NaCl is is 10 10 wt.% wt.% (a (),a), 20 20 wt.% wt.% (b (),b), and and 30 30 wt.% wt.% (c ().c). Figure 3 shows the SEM images of the internal pore structure of the 30 wt.% NaCl samplesFigureFigure with 33 shows showszinc acetate the the SEM SEM (Figure images images 3a,d), of of the zincthe internal internalsulfate pore (Figurepore structure structure 3b,e), of and the of 30zincthe wt.% 30 chloride wt.% NaCl NaCl sam-(Fig- samplesureples 3c,f) with aswith zinc an zincelectrolyte. acetate acetate (Figure The (Figure3 a,d),top row3a,d), zinc in sulfate zincFigu resulfate (Figure 3a–c can(Figure3b,e), be andused 3b,e), zinc to and evaluate chloride zinc chloridethe ( Figure deposited (Fig-3c,f ) urezincas an 3c,f) amount electrolyte. as an inside electrolyte. The the top samples. The row top in It row Figure is noteworthy in Figu3a–cre can 3a–c that be can used SEM be to usedimages evaluate to evaluateonly the capture deposited the deposited a limited zinc zincareaamount amountof the inside entire inside the sample. the samples. samples. However, It isIt is noteworthy basednoteworthy on a thatthorough that SEM SEM imagesSEM images analysis only only capturecapturealso displayed aa limited in areaFigurearea of of 3,the it entire can be sample. concluded However, However, that zinc based acet ateon resulteda thorough in a SEM lower analysis deposited also zinc displayed amounts in FigurecomparedFigure 33,, itit to cancan the bebe other concludedconcluded two electrolytes. thatthat zinczinc acetateacet Theate dep resultedresultedosited inzincin aa lowerloweramount depositeddeposited was highest zinczinc amountswithamounts zinc comparedchloridecompared electrolyte. to to the the other other two two electrolytes. The The dep depositedosited zinc amount was highest with zinc chloridechlorideThere electrolyte. electrolyte. is a clear difference in the deposited zinc morphology depending on the used electrolyte,There isas a shown clear difference in the high in magnification the deposited SEM zinc imagesmorphology in Figure depending 3d–f. The on deposited the used electrolyte, as shown in the high magnification SEM images in Figure 3d–f. The deposited

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Coatings 2021, 11, 949 zinc does not form crystals with sharp edges using zinc acetate (Figure 3d), whereas6 ofthe 11 deposited crystals are thin, leaf-like, and more regular in shape and size with zinc sulfate (Figure 3e). The most regular crystal shapes were achieved with zinc chloride electrolyte (Figure 3f) having thin and hexagonal shaped crystals. TheThere observed is a clear differences difference both in the in depositedquantity and zinc quality morphology of the deposited depending zinc on may the usedorig- inateelectrolyte, from the as showndifferences in the in high the electric magniﬁcation current SEM during images electrolysis in Figure with3d–f. different The deposited electrolytes.zinc does For reproducibility not form crystals and with comparability sharp edges of using each zinctest, acetatethe power (Figure supply3d), was whereas operated the indeposited a constant crystals voltage are mode thin, leaf-like,at 2.0 V. andThe moreother regularparameters, in shape such and as the size distance with zinc between sulfate the(Figure anode3e). and The cathode most regular and the crystal electrolyte shapes wereconcentration, achieved withwere zinckept chloride constant. electrolyte It can be concluded(Figure3f) havingthat the thin varying and hexagonalmorphologies shaped resu crystals.lted from the differences in electrolyte com- poundsThe as observed observed differences in the varying both electric in quantity current and during quality the of electrolysis the deposited from zinc around may 0.04originate A for fromzinc theacetate differences to 0.05 A in for the zinc electric sulfate current and 0.08 during A for electrolysis zinc chloride. with A different higher electricalelectrolytes. conductivity For reproducibility of the zinc and chlori comparabilityde electrolyte of eachresulted test, in the the power observed supply higher was amountoperated of in electrodeposited a constant voltage zinc. mode This at is 2.0 in V. agreement The other with parameters, the literature such as [36] the as distance coarse crystalsbetween of the zinc anode were and obtained cathode from and theacid electrolyte sulfate solution concentration, electrodeposition, were kept constant. and even It coarsercan be concluded crystals resulted that the from varying electroplating morphologies with resulted chloride from solution. the differences in electrolyte compoundsAs a general as observed remark, in a the regional varying variation electric currentin the shape during and the size electrolysis of the zinc from deposits, around as0.04 well A as for the zinc quantity acetate of to deposited 0.05 A for zinc, zinc was sulfate observed and 0.08irrespective A for zinc of the chloride. used electrolyte. A higher Typically,electrical conductivitya larger deposited of the zinc chlorideamount was electrolyte observed resulted closer in to the the observed sample surface higher amount of electrodeposited zinc. This is in agreement with the literature [36] as coarse where the electric field was the greatest. It is also important to note that in all cases the crystals of zinc were obtained from acid sulfate solution electrodeposition, and even coarser pores and cavities were not completely filled with zinc. However, the steel surface, even crystals resulted from electroplating with chloride solution. inside the samples, was coated with zinc deposits with varying thicknesses and amounts.

Figure 3. High magnification magnification SEM images of the internal pore structurestructure of sintered 440C stainless steel samples after zinc electrodeposition. The amount of NaCl is 30 wt.%wt% in in all all imag images.es. The The electrolytes electrolytes used used in in the electrodeposition process are zinc acetate (a,d), zinc sulfate (b,e), and zinc chloride (c,f). zinc acetate (a,d), zinc sulfate (b,e), and zinc chloride (c,f). FigureAs a general 4 shows remark, an EDS a regional analysis variation of the deposited in the shape zinc andinside size a ofporous the zinc 440C deposits, stainless as steelwell assample the quantity (30 wt.% of NaCl) deposited in the zinc,formwas of an observed X-ray line irrespective scan with zinc of the acetate used electrolyte. TheTypically, amount a larger of zinc deposited (blue) was zinc compared amount was to observedthe amount closer of iron to the (green) sample and surface chromium where (red).the electric The curve field wasof zinc the coincides greatest. It with is also the important occurrence to of note the that white in all structures cases the in pores the andsec- ondarycavities electron were not image completely of the filled sample. with Thus zinc., EDS However, analysis the confirmed steel surface, that even the insideobserved the samples, was coated with zinc deposits with varying thicknesses and amounts. Figure4 shows an EDS analysis of the deposited zinc inside a porous 440C stainless steel sample (30 wt.% NaCl) in the form of an X-ray line scan with zinc acetate electrolyte. The amount of zinc (blue) was compared to the amount of iron (green) and chromium (red). Coatings 2021, 11, 949 7 of 11

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The curve of zinc coincides with the occurrence of the white structures in the secondary electron image of the sample. Thus, EDS analysis conﬁrmed that the observed white ﬂakes insidewhite theflakes samples inside were the zinc.samples It is were also worthzinc. It emphasizing is also worth the emphasizing presence of zinc the throughoutpresence of thezinc scanned throughout line inthe smaller scanned amounts, line in smaller which demonstratesamounts, which the demonstrates deposition of the zinc deposition over the entireof zinc surface over the as entire a thin surface layer. Inas addition,a thin layer. the In oxygen addition, signals the inoxygen the EDS signals analysis in the were EDS observedanalysis were to coincide observed with to the coincide zinc peaks, with whichthe zinc suggests peaks, thewhich deposition suggests of the zinc deposition as an oxide of intozinc the as structure.an oxide into It is the known structure. from the It is EDS known analysis from literature the EDS that analysis the nominal literature difference that the betweennominal Odifference Kα and Crbetween Lα peaks O Kα is and only Cr 51 L eV,α peaks and thusis only the 51 overlap eV, and of thus these the peaks overlap may of resultthese inpeaks the may incorrect result detection in the incorrect of oxygen detection in the sampleof oxygen by in the the detector. sample Forby the the detector. sake of clarity,For the the sake oxygen of clarity, peaks the are oxygen not displayed peaks are in not Figure displayed4 as the in main Figure emphasis 4 as the was main to empha- verify zincsis was deposition to verify into zinc the deposition structure. into the structure.

Figure 4. SEM-EDS line scan (along the yellow dashed line on the left image) chemical composition analysis (right image) Figure 4. SEM-EDS line scan (along the yellow dashed line on the left image) chemical composition analysis (right image) insideinside a a porous porous 440C 440C stainless stainless steel steel sample sample with with 30 30 wt.% wt% porosityporosity andand zinczincacetate acetate used used as as an an electrolyte. electrolyte.

3.3.3.3. CorrosionCorrosion TestingTesting FigureFigure5 5 shows shows the the imagesimages ofof thethe 30 wt.% po porosityrosity samples samples after after the the corrosion corrosion test test in inneutral neutral salt salt spray spray for the for durati the durationon of 240 of h. 240The h.effect The of effectelectrolyte of electrolyte composition composition on pitting oncorrosion pitting behavior corrosion can behavior be clearly can observed. be clearly Each observed. row shows Each four row parallel shows samples four parallel from samplesboth sides from with both different sides with electrolytes. different The electrolytes. left half of The each left row, half ofi.e., each the row,first i.e.,four the images first fouron each images row on displays each row the displays side of the the samples side of thethat samples faced the that zinc faced anode the during zinc anode electrolysis. during electrolysis.On the right Onside the are right the same side aresamples the same from samples the different from side. the different The reference side. The 440C reference samples 440Cwithout samples zinc plating without on zinc the plating top row on (Figure the top 5a) row display (Figure significant5a) display corrosion. significant A corrosion.significant Aimprovement significant improvement in corrosion resistance in corrosion was resistance observed was with observed the electroplated with the electroplatedsamples. The samples.lowest corrosion The lowest was corrosion observed was for observedzinc acetate for zinc(Figure acetate 5b) and (Figure zinc5b) sulfate and zinc (Figure sulfate 5c), (Figurewhereas5c), zinc whereas chloride zinc electrolyte chloride (Figure electrolyte 5d) (Figuredisplayed5d) extensive displayed corrosion extensive on corrosion both sides onof boththe sample. sides of This the sample.was expected This wasdue expectedto the corrosive due to thenature corrosive of the natureacidic chloride of the acidic elec- chloridetrolyte [36]. electrolyte The highly [36]. corrosive The highly environment corrosive environmentof the electrolysis of the bath electrolysis and the bathcorrosion and theprone corrosion grade 440C prone stainless grade 440C steel stainless caused observ steel causedable corrosion observable in the corrosion samples in after the samples electrol- after electrolysis and before the salt spray testing. This initial corrosion explains the ysis and before the salt spray testing. This initial corrosion explains the observed extensive observed extensive corrosion of zinc chloride samples in the salt spray test despite the corrosion of zinc chloride samples in the salt spray test despite the larger deposited zinc larger deposited zinc amount compared to the zinc acetate and zinc sulfate samples. amount compared to the zinc acetate and zinc sulfate samples.

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Figure 5. Images of porous 440C stainless steel samplessamples withwith 3030 wt.%wt% porosity after the 240 h corrosion test. References without zinc (a), zinc acetate electrolyte (b), zinc sulfate electrolyte (c), and zinc chloride electrolyte (d) are displayed from without zinc (a), zinc acetate electrolyte (b), zinc sulfate electrolyte (c), and zinc chloride electrolyte (d) are displayed from both sides (the anode side and the opposite side relative to the anode) of the sample. both sides (the anode side and the opposite side relative to the anode) of the sample.

InIn summary, summary, zinc zinc coating coating improved improved pitting pitting corrosion corrosion resistance resistance for each for eachsample sample com- paredcompared to the to thereference. reference. It is It noteworthy is noteworthy that that the the side side facing facing away away from from the the zinc zinc anode (labeled(labeled opposite opposite side side on on the the right right side side of of b b and and c) c) corroded corroded significantly signiﬁcantly less than the side facingfacing the the zinc zinc anode anode (labeled (labeled anode anode side side on on th thee left left side side of of b and c). This may be due to thethe electrolyte electrolyte bath bath setup, setup, i.e., i.e., the the size size and and sh shapeape of of the the used used beaker beaker in in electrolysis electrolysis and the mixingmixing speed speed may may have have had had an an effect on the uniformity of the zinc plating. Finally,Finally, Figure Figure 6 shows the effect effect of of porosi porosityty on on corrosion corrosion behavi behavior.or. Porosities Porosities of of10 wt.%10 wt.% (Figure (Figure 6a,b),6a,b), 20 20 wt.% wt.% (Figure (Figure 6c,d),6c,d), and and 30 30 wt.% (Figure 6 6e,f)e,f) wt.%wt.% of of NaCl NaCl were were usedused to to create create different different porosities, porosities, and and zinc zinc electrodepositions electrodepositions (Figure 66b,d,f)b,d,f) werewere carriedcarried outout by using zinczinc acetateacetate electrolyte.electrolyte. Reference Reference 440C 440C samples samples displayed displayed heavy heavy corrosion corrosion on onboth both sides, sides, as shown as shown in Figure in Figure6a,c,e 6a,c,e (similar (sim toilar Figure to Figure5). For all5). porosities,For all porosities, the zinc coatingthe zinc coatingimproved improved corrosion corrosion resistance resistance compared compar to theed referenceto the reference 440C. However,440C. However, no signiﬁcant no sig- nificantdifferences differences in corrosion in corrosion resistance resistance were observed were observed between differentbetween porositiesdifferent porosities with zinc withcoated zinc samples. coated Forsamples. lower For porosities lower porosities shown in shown Figure 6inb,d, Figure the differences 6b,d, the differences in corrosion in corrosionbetween the between two sides the oftwo the sides samples of the were samples not as large were as not in theas large caseof as the in highestthe case porosity of the highest(Figure 6porosityf). (Figure 6f).

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Figure 6. Images ofof porousporous 440C440C stainlessstainless steelsteel samplessamples withwith varyingvarying porosityporosity of of 10 10 wt.% wt% ((aa,b,b),), 2020 wt.%wt% ((cc,,dd),), andand 3030 wt.%wt% (e,f) after the 240 h corrosion test. Reference samples without zinc (a,c,e) and the corresponding corrosion protected sam- (e,f) after the 240 h corrosion test. Reference samples without zinc (a,c,e) and the corresponding corrosion protected samples ples with zinc acetate electrolyte (b,d,f) are displayed with both sides (the anode side and the opposite side relative to the with zinc acetate electrolyte (b,d,f) are displayed with both sides (the anode side and the opposite side relative to the anode) anode) of the sample. Images of the internal pore structure of the sintered 440C stainless steel samples. The amount of ofNaCl the is sample. 10 wt% Images (a), 20 wt% of the (b internal), and 30 pore wt% structure (c). of the sintered 440C stainless steel samples. The amount of NaCl is 10 wt.% (a), 20 wt.% (b), and 30 wt.% (c).

4. Conclusions Conclusions Porous injection molded 440C stainless steel structures were fabricated by using the PSH technique, technique, and and the the internal internal pore pore struct structureure was was electrochemically electrochemically deposited deposited with with corrosioncorrosion protective protective zinc zinc by three by three different different electr electrolyteolyte compositions compositions of zi ofnc zincacetate, acetate, zinc zincsul- fate,sulfate, and and zinc zinc chloride. chloride. The The used used electrolyte electrolyte so solutionlution was was observed observed to to have have a a significant signiﬁcant effect on the morphology and the amount of of th thee deposited zinc zinc inside inside the the pore pore structure. structure. The most regular zinc crystals and the thicke thickestst coating were observed with zinc chloride, whereas the the lowest lowest deposited deposited zinc zinc amount amount with with the the most most irregular irregular zinc zinc crystals crystals was was ob- servedobserved with with zinc zinc acetate. acetate. The objective objective of of this this study study was was to to demonstra demonstratete a cathodic a cathodic corrosion corrosion protection protection of in- of jectioninjection molded molded porous porous 440C 440C stainless stainless steel steel structures structures by by electroplating electroplating zinc zinc into into the the pores. pores. Corrosion protection was achieved by zinc coatingcoating of 440C stainless steel samples. It can be concluded that electrodeposition from zinc acetate electrolyte resultedresulted in the highest pitting corrosioncorrosion resistance.resistance. OnOn the the contrary, contrary zinc, zinc chloride chloride resulted resulted in thein the weakest weakest corrosion corro- sionprotection. protection. The The porosity porosity of the of samplesthe samples was was not not observed observed to have to have a signiﬁcant a significant effect effect on oncorrosion corrosion resistance. resistance. It is It believed is believed that that the resultsthe results observed observed here here can becan applied be applied to enhance to en- hancethe corrosion the corrosion protection protection of stainless-steel of stainless-st grades.eel grades. Furthermore, Furthermore, it is expectedit is expected that zincthat zincplating plating incorporated incorporated into theinto pore the structurepore struct ofure porous of porous stainless-steel stainless-steel materials materials may enable may enable the use of such porous structures in more corrosive and environmentally harsh conditions in the future.

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the use of such porous structures in more corrosive and environmentally harsh conditions in the future.

Author Contributions: M.K.: investigation and writing—original draft; K.M.: conceptualization, funding acquisition, project administration, and resources; J.J.S.: conceptualization, project administration, supervision, and writing—review and editing; M.S.: conceptualization, funding acquisition, project administration, resources, supervision, and writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Business Finland/ERDF (European Regional Develop- ment Fund) project “MIM Components for Harsh Conditions” (Grant agreement 7929/31/2019) for financial support. J.J.S. acknowledges the Faculty of Science and Forestry at the University of Eastern Finland for financial support (grant no. 579/2017) and the Academy of Finland Flagship for Photonics Research and Innovation (PREIN, decision no. 320166). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in article here. The data presented in this study are also available upon request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

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