Pitting Corrosion in AISI 304 Rolled Stainless Steel Welding at Different Deformation Levels

applied sciences

Article Pitting Corrosion in AISI 304 Rolled Stainless Steel Welding at Diﬀerent Deformation Levels

Francisco-Javier Cárcel-Carrasco 1 , Manuel Pascual-Guillamón 1 , Lorenzo Solano García 2,*, Fidel Salas Vicente 1 and Miguel-Angel Pérez-Puig 1

1 ITM, Universitat Politècnica de València, 46022 Valencia, Spain 2 Departamento de Ingeniería Mecánica y de Materiales, Universitat Politècnica de València, 46022 Valencia, Spain * Correspondence: [email protected]; Tel.: +34-963-87-7000 (ext. 76273); Fax: +34-963-87-7627

Received: 12 July 2019; Accepted: 7 August 2019; Published: 9 August 2019

Abstract: This paper analyzes pitting corrosion at the weld zone and at the heat aﬀected zone (HAZ) in AISI 304 rolled stainless steel welds. As the aforementioned material is one of the most frequently used types of stainless steel, it is needful to be aware of the mechanisms that lead to its deterioration, like corrosion, since it can cause failures or malfunction in a wide variety of products and facilities. For the experimental tests 1.5 mm thick AISI 304 stainless steel plates were welded and rolled to diﬀerent thicknesses and after, the samples were subjected to mechanical and corrosion tests and to a micrograph study. Deformation stresses and other intrinsic metallurgic and physic-chemical transformations that occur during cold rolling and welding, and that are key factors in the anti-corrosion behavior of AISI 304 rolled stainless steel, have been observed and analyzed. A correlation has been found between cold work levels in test samples and number of pits after corrosion tests.

Keywords: pitting corrosion; welding; cold rolling; deformation stress; AISI 304; AISI 308L

1. Introduction The objective of this study is to analyze the resistance of AISI 304 welds to pitting corrosion when they have been subjected to diﬀerent levels of cold rolling, as well as to establish a correlation between the pitting corrosion and cold rolling deformation level. The pitting corrosion, initiated at certain points of the surface due to the non-uniformity of the passive layer, is a localized redox (reduction-oxidation) process that occurs inside small pits on the surface of passivated metals in aqueous media. This kind of reaction requires the balancing of both the reduction and oxidation half-reactions. The anodic reaction that takes place inside the pit leads to the dissolution of iron (Fe Fe2+ + 2e ) while the cathodic reaction, that makes use of the electrons → − liberated by the anodic reaction, leads to the apparition of hydroxide (1/2O + H O + 2e 2(OH )). 2 2 − → − As inside the pit, the electrolyte solution becomes positively charged due to the presence of Fe2+ ions, the anionic species (Cl−) in the electrolyte are attracted there, increasing the acidity inside the pit and accelerating the corrosion process according to the reactions:

FeCl Fe3+ + 3Cl (outside the pit) 3 → − Fe3+ + 3(OH) Fe(OH) (outside the pit) − → 3 Fe2+ + 2Cl FeCl (at the pit) − → 2 FeCl + 2H O Fe(OH) + 2HCl (at the pit) 2 2 → 2

Appl. Sci. 2019, 9, 3265; doi:10.3390/app9163265 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 3265 2 of 12

Furthermore, the solid Fe(OH)3 deposited around the pit increases the separation of the inside of the pit from the rest of the electrolyte. The presence of chemical species that provide a high concentration of Cl− in the aqueous media is needed to accelerate the process of corrosion in the pit. AISI 304 stainless steel is widely used in the manufacturing of both industrial and domestic products, mainly due to the combination of characteristics that presents: Good mechanical properties (high strength, ductility and malleability); non-magnetic behavior; weldability; resistance to corrosion and low cost [1–4]. It is used in applications that entail a high stress level and/or exposure to corrosive atmospheres, like in nuclear power plants [5,6], marine environments [7,8] or in large facilities [9]. Particularly, AISI 304 rolled stainless steel is one of the most frequently used types of stainless steel, whereby it is needful to be aware of the mechanisms that lead to its deterioration in order to foresee, restrict and/or quantify the economic costs that could be derived. Corrosion is one of the key factors to consider, as it can cause failures or malfunction in all types of products and facilities. Specially, when they get in contact with corrosive agents or when they are used in conditions or participate in processes that bring on corrosion [8], like welding and cold forming. The anti-corrosion behavior of welded stainless steels has been studied previously [10]. AISI 304 stainless steel can be welded by almost all the common welding techniques [11]. In arc welding the use of a low carbon content stainless steel as a filler rod reduces chromium carbide, which improves corrosion resistance [7,12]. However, corrosion resistance can be affected when stainless steel is subjected to cold rolling [13,14]. The friction and compression forces under conditions of dynamic plastic deformation cause the material fibers to stretch while its stress level rises and its hardness increases [15]. Beyond a certain level of deformation different types of defects and even failure can occur. If the process is under control these defects can be avoided, but the relieve of internal stresses requires an annealing treatment [16]. Alike, when materials obtained by cold rolling are welded, the stress state changes due to the effect of heat and the apparition of new stresses during the cooling of the weld, especially if deformation is impeded. These welding stresses depend on the energy applied and the heat transmission conditions during the welding process [17]. Cold rolling and welding are part of various manufacturing processes. So, the combined effect of both in the stresses increment, and its influence in corrosion, can be present in many manufactured products. The effect of the residual stresses generated during products manufacturing of stainless steel on the stress corrosion cracking susceptibility is presented by [18]. Other works listed below focused on the pitting corrosion, the factors that cause it and their effects: In [19] a summary of the effects of possible factors in the pitting corrosion of metals is given; in [20] cold working was discussed as one factor on the mechanism and rate of pitting corrosion; in [21] the pitting generation caused by applied stress has been studied; in [22] an overview of the critical factors influencing the pitting corrosion of metals is provided; in [23] a method of evaluating strength reduction due to the pitting corrosion is established; in [24] a quantitative analysis of the effect of Type I residual stresses on the occurrence of pitting and stress corrosion is investigated; in [25] the effects of the micro-plasma arc welding technique on the pitting corrosion of different zones of an AISI 316L stainless steel were studied; in [12] the effect of low intensity X-rays ionizing radiation on TIG-welds of AISI 304 stainless steel using AISI 316L as filler rods is analysed. In this work, a batch of welded samples of AISI 304 stainless steel were subjected to different levels of cold work to change the shape of grains, create residual stresses and damage the surface. The influence of those changes in the pitting corrosion resistance of the welds was evaluated by the immersion of the samples in a FeCl3 solution according to a standardized test and quantified by the pits number [8,26–29]; while deformation levels were measured as the thickness reduction of the metal samples (cold work level). A mathematical relation between the cold work level and the number of pits is proposed for the tested samples. Appl. Sci. 2019, 9, x 3265 FOR PEER REVIEW 33 of of 1213 traction and stamping. The filler metal was low-carbon ER AISI 308L, which reduces the formation 2.of Materialschromium and carbide Experimental in the grain Procedures boundary during welding and is recommended for TIG or MIG (metal inert gas) welding of the AISI 304 steel. The AISI 308L has good corrosion resistance, is non- 2.1. Characteristics of Materials Used magnetic, and is principally used in the chemical industry and food applications. Its composition is also shownThe base in materialTable 1. of the welded joint (AISI 304), whose chemical composition is given in Table1, is a non-magnetic weldable austenitic stainless steel with good mechanical characteristics, highly resistantTable to corrosion, 1. Chemical which composition does not of hardenAISI 304 inand heat ER AISI treatments 308 L base and and easily filler deformablestainless steels. by rolling, traction and stamping. The filler metal was low-carbon ER AISI 308L, which reduces the formation of Material %C %Si %Mn %Cr %Ni %Mo %Fe chromium carbide in the grain boundary during welding and is recommended for TIG or MIG (metal AISI 304 0.07 1 inert gas) welding of the AISI 304 steel. The AISI 308L2 has good19 corrosion11 resistance,- is non-magnetic,Bal. and is principallyER AISI used308L in the 0.02 chemical 0.38 industry and1.90 food19.80 applications. 9.8 Its composition 0.19 Bal.is also shown in Table1. 2.2. Tests Description Table 1. Chemical composition of AISI 304 and ER AISI 308 L base and filler stainless steels. 2.2.1. Sample Preparation Material %C %Si %Mn %Cr %Ni %Mo %Fe WeldingAISI 304 was carried 0.07 out on a horizontal 1 plane 2 in an inert 19 argon atmosphere 11 -and employing Bal. non- consumableER AISI tungsten-thorium 308L 0.02 alloy 0.38 electrode. 1.90 The welding 19.80 parameters 9.8 were: Gun 0.19 nozzleBal. exit rate of 11 dm3/min, current 48 A, applied bias from 10 to 12 V, travel rate of 50 mm/min, torch inclination 2.2.angle Tests from Description 70° to 80° and filler rod inclination angle of 15°. Stainless steel experiments more heat deformation than other carbon steels or steel alloys. So, to 2.2.1.avoid Samplesignificant Preparation deformation during the welding process, the plates were fixed until they returned to theWelding ambient was temperature. carried out However, on a horizontal the undesire plane ind effect an inert of argona small atmosphere residual stress and employingdue to the non-consumabledeformation restriction tungsten-thorium during cooling alloy is electrode. expected The[30]. welding parameters were: Gun nozzle exit rate of 11The dm 3welds/min, currentwere carried 48 A, out applied on twelve bias from AISI 10304 to steel 12 V, plates travel measuring rate of 50 mm200 /mmmin, in torch length, inclination 100 mm in width, and 1.5 mm thick. Once welded, the plates were cut perpendicularly to the weld bead at 20 angle from 70◦ to 80◦ and filler rod inclination angle of 15◦. mm intervalsStainless to steel obtain experiments 10 samples more from heat each deformation plate; 60 samples than other in total carbon measuring steels or100 steel mm alloys. in length, So, to20 avoidmm in significant width, and deformation 1.5 mm thick during (see Figure the welding 1). The process, joints were the plates smoothed were fixedand polished, until they removing returned tosurface the ambient irregularities temperature. to obtaining However, the same the thickness undesired as the effect original of a smallplates. residual stress due to the deformationThese samples restriction were during cold rolled cooling to isobtain expected five [different30]. thickness batches: 12 samples of 1.5 mm thick,The 12 weldssamples were of 1.25 carried mm out thick, on twelve 12 samples AISI 304of 1 steel mm plates thick,measuring 12 samples 200 of mm0.75 inmm length, thick 100 and mm 12 insamples width, of and 0.65 1.5 mm mm thick. thick. The Once samples welded, thickness the plates of each were batch cut is perpendicularly expressed as “e” to in the Figure weld 1. bead Those at 20values mm correspond intervals to obtainto cold 10 work samples percentages from each of plate;0%, 17%, 60 samples 33%, 50% in and total 57%, measuring respectively. 100 mm in length, 20 mmSubsequently, in width, and the 1.5 samples mm thick selected (see Figurefor corrosion1). The tests joints were were cut smoothed to 40 mm and in length, polished, leaving removing welds surfaceapproximately irregularities in the tocenter obtaining (Figure the 1). same thickness as the original plates.

Figure 1. Samples preparation for tests: Number of samples (in brackets), samples dimensions (in mm), Figure 1. Samples preparation for tests: Number of samples (in brackets), samples dimensions (in and preparation operations and tests to be carried out. mm), and preparation operations and tests to be carried out. These samples were cold rolled to obtain ﬁve diﬀerent thickness batches: 12 samples of 1.5 mm thick,2.2.2. Micrographs 12 samples ofand 1.25 Hardness mm thick, Tests 12 samples of 1 mm thick, 12 samples of 0.75 mm thick and Appl. Sci. 2019, 9, 3265 4 of 12

12 samplesAppl. Sci. of 0.652019, mm9, x FOR thick. PEER The REVIEW samples thickness of each batch is expressed as “e” in Figure1. Those 4 of 13 values correspond to cold work percentages of 0%, 17%, 33%, 50% and 57%, respectively. Subsequently,Two samples the samples from each selected batch for were corrosion selected tests for were micrographic cut to 40 mm tests in length,(Figure leaving1). One welds to obtain a approximatelysurface micrograph in the center and (Figure the other1). for a transversal micrograph. The surface micrographs allowed the study of the changes that take place at the surface and that could lead to a deterioration of the 2.2.2. Micrographs and Hardness Tests protective passive layer on the stainless plate. The transversal micrographs provide information Twoabout samples the welding from each microstructure batch were and selected how for the micrographic grains are deformed tests (Figure by the1). Onerolling to obtainprocess, a as the surfacechanges micrograph in the andmicrostructure the other for driven a transversal by plastic micrograph. deformation The could surface affect micrographs the corrosion allowed resistance of the studythe joints. of the changes that take place at the surface and that could lead to a deterioration of the protective passiveFor the layermicrographs, on the stainless the samples plate. Thewere transversal cut as indi micrographscated in Figure provide 2. After information cutting, about they were the weldingpolished microstructure in two stages: and First how a thegrinding grains with are deformed 200 and by500 the grain rolling size process, abrasive as paper, the changes followed in by a the microstructurefinal polishing driven with by3 μ plasticm and deformation1 μm diamond could paste. aﬀ ectFinally, the corrosion the samples resistance were electroetched of the joints. in a 10% Foroxalic the acid micrographs, solution. the samples were cut as indicated in Figure2. After cutting, they were polished inThe two same stages: samples First a grindingwere used with to measure 200 and 500the grainhardness size at abrasive the weld paper, bead, followed the HAZ by and a ﬁnal the base polishingmetal. with 3 µm and 1 µm diamond paste. Finally, the samples were electroetched in a 10% oxalic acid solution.

(Rolling direction)

(a) First cut in test sample

(b) Second cut in test sample

1 Welded zone 2 HAZ (c) Transversal section after second cut 3 Base material 1 3 2

Figure 2. Samples preparation for micrographs and hardness measurement. The dashed triple line representsFigure the 2. rolling Samples direction. preparation for micrographs and hardness measurement. The dashed triple line represents the rolling direction. The same samples were used to measure the hardness at the weld bead, the HAZ and the base metal.The micrographs were made with an Optikam camera (Optika, Ponteranica, Italy) software in a TheNIKON micrographs metallographic were made microscope with an OptikamMicrophot-FX camera (Nik (Optika,on, Tokio, Ponteranica, Japan). For Italy) microhardness software in testing, a NIKONa Vickers metallographic micro-durometer microscope INNOVATEST Microphot-FX 400A (Nikon, (Innov Tokio,atest, Japan). Townley, For The microhardness Netherlands) testing, with 300 g a Vickersload micro-durometer was used and 12 INNOVATESTs of dwell time. 400A (Innovatest, Townley, The Netherlands) with 300 g load was used and 12 s of dwell time. 2.2.3. Tensile Tests 2.2.3. Tensile Tests Ultimate and yield strengths were evaluated by tensile tests performed on five samples from Ultimateeach batch and (Figure yield strengths 1) using werea 100 evaluated kN universal by tensile test machine tests performed Electrotest on ﬁveMD-2 samples (Ibertest, from Daganzo each de batch (FigureArriba,1 Spain),) using at a 100a test kN speed universal of 10 testmm/min. machine Electrotest MD-2 (Ibertest, Daganzo de Arriba, Spain), at a test speed of 10 mm/min. 2.2.4. Corrosion Tests 2.2.4. Corrosion Tests Five samples from each batch were selected for corrosion tests (Figure 1). These tests, in Five samples from each batch were selected for corrosion tests (Figure1). These tests, in accordance accordance with ASTM G48-92 [31], consisted in submerging the samples during 72 h in a 10% mass with ASTM G48-92 [31], consisted in submerging the samples during 72 h in a 10% mass (0.387 M) (0.387 M) solution of iron(III) chloride hexahydrate (FeCl3·6H2O) electrolyte and the subsequent solution of iron(III) chloride hexahydrate (FeCl3 6H2O) electrolyte and the subsequent evaluation of evaluation of the pitting corrosion in the weld· and HAZ zones. The FeCl3 provide the Cl− ions needed the pittingto accelerate corrosion the in corrosion the weld andrate HAZas described zones. The in the FeCl introduction.3 provide the Cl− ions needed to accelerate the corrosion rate as described in the introduction. 3. Results

3.1. Micrographs and Hardnesses Appl. Sci. 2019, 9, 3265 5 of 12

3.Appl. Results Sci. 2019, 9, x FOR PEER REVIEW 5 of 13 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 13 3.1.Appl. Micrographs FiguresSci. 2019, 93–7, x andFOR show HardnessesPEER the REVIEW transversal micrographs performed at the fusion line of welded zones5 of at13 samples of 1.5 mm to 0.65 mm thick. Figures 3–7 show the transversal micrographs performed at the fusion line of welded zones at FiguresInFigures these3– micrographs3–77 show show the the transversal the transversal granular micrographs micrographsdeformation performed performeddue to rolling at at the thecan fusion fusionbe seen line line at of bothof welded welded the weld zones zones bead at at samples of 1.5 mm to 0.65 mm thick. samplesandsamples the ofHAZ. of 1.5 1.5 mm Asmm tothe to 0.65 deformation0.65 mm mm thick. thick. occurred in a cold forming process, the structures changed from In these micrographs the granular deformation due to rolling can be seen at both the weld bead dendriticInIn these these at micrographsthe micrographs weld bead the the to granular granularequiaxial deformation deformation at the HAZs due due and to to rollingthe rolling base can can metal. be be seen seen Those at at both bothstructures the the weld weld become bead bead and the HAZ. As the deformation occurred in a cold forming process, the structures changed from andclearlyand the the elongated HAZ. HAZ. As As theas the the deformation deformation thickness re occurred occurredduction inincreases. in a a cold cold forming Thisforming difference process, process, is the progressivelythe structures structures changed reduced changed fromas from the dendritic at the weld bead to equiaxial at the HAZs and the base metal. Those structures become dendriticdeformationdendritic at at the thedegree weld weld by bead bead rolling to to equiaxial increases,equiaxial at asat the canthe HAZs HAZsbe seen and and in theFigures the base base 3–7. metal. metal. Those Those structures structures become become clearlyclearlyclearly elongated elongatedelongated as asas the thethe thickness thicknessthickness reduction rereductionduction increases. increases.increases. This ThisThis di differencedifferenceﬀerence is isis progressively progressivelyprogressively reduced reducedreduced as asas the thethe deformationdeformationdeformation degree degreedegree by byby rolling rollingrolling increases, increases,increases, as asas can cancan be bebe seen seenseen in inin Figures FiguresFigures3– 3–7.73–7..

Figure 3. Transversal micrography of the non-rolled sample (1.5 mm thick) at the weld bead (right)/heat affected zone (HAZ) (left) interface. FigureFigureFigure 3. 3.3.Transversal TransversalTransversal micrography micrographymicrography of the ofof non-rolled thethe non-rollednon-rolled sample samplesample (1.5 mm (1.5(1.5 thick) mmmm at thethick)thick) weld atat bead thethe (right) weldweld / heatbeadbead a(right)/heatﬀ(right)/heatected zone affectedaffected (HAZ) (left) zonezone interface. (HAZ)(HAZ) (left)(left) interface.interface.

Figure 4. Transversal micrography at the weld bead (right)/HAZ (left) interface after 17% cold rolling. Figure 4. Transversal micrography at the weld bead (right)/HAZ (left) interface after 17% cold rolling. FigureFigure 4.4. TransversalTransversal micrographymicrography atat ththee weldweld beadbead (right)/HAZ(right)/HAZ (left)(left) interfaceinterface afterafter 17%17% coldcold rolling.rolling.

FigureFigure 5. Transversal 5. Transversal micrography micrography at the at weld the weld bead bead (right) (right)/HAZ/HAZ (left) (left) interface interface after 33%aftercold 33% rolling.cold rolling. FigureFigure 5.5. TransversalTransversal micrographymicrography atat ththee weldweld beadbead (right)/HAZ(right)/HAZ (left)(left) interfaceinterface afterafter 33%33% coldcold rolling.rolling. Appl. Sci.Appl.2019 Sci., 20199, 3265, 9, x FOR PEER REVIEW 6 of 136 of 12 Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 13

Figure 6. Transversal micrography at the weld bead (right)/HAZ (left) interface after 50% cold Figure 6.FigureTransversal 6. Transversal micrography micrography at the at weld the weld beadrolling. bead (right) (right)/HAZ/HAZ (left) (left) interface interface after after 50%50% cold rolling. rolling.

Figure 7.FigureTransversal 7. Transversal micrography micrography at the at weld the weld bead bead (right) (right)/HAZ/HAZ (left) (left) interface interface after after 57%57% cold rolling. Figure 7. Transversal micrography at the weldrolling. bead (right)/HAZ (left) interface after 57% cold Figure8 shows the presence of deformationrolling. induced martensite at the samples after deformation. AlthoughFigure the tests 8 wereshows carried the presence out at room of deformation temperature induced and martensite martensite formation at the issamples favored after by lower deformation.Figure 8 Although shows thethe testspresence were ofcarried deformation out at room induced temperature martensite and martensiteat the samples formation after is temperatures, the cold work level is high enough to compensate for the higher temperatures of the favoreddeformation. by lower Although temperatures, the tests werethe cold carried work out leve at lroom is high temperature enough to and compensate martensite for formation the higher is rolling. As stated by [32], the presence of this martensite has a great inﬂuence on the performance of temperaturesfavored by lower of the temperatures, rolling. As stated the coldby [32], work the leve presencel is high of thenoughis martensite to compensate has a great for influence the higher on the protective layer that covers the deformed samples. thetemperaturesAppl. performance Sci. 2019, 9, ofx FOR the of thePEERrolling. protective REVIEW As stated layer by that [32], covers the presence the deformed of this samples.martensite has a great influence7 of on 13 the performance of the protective layer that covers the deformed samples.

FigureFigure 8. Deformation 8. Deformation twins twins in in one one of of the the 33%33% coldcold rolled samples. samples. Martensite Martensite grows grows along along the the deformationdeformation twin twin boundaries. boundaries.

The high levels of cold work also generated small cracks at the surface of the welded samples (Figure 9). This leads to a decrease in the performance of the protective layer as its continuity is broken and a slight reduction of the mechanical properties when compared to a sample with no cracks.

Figure 9. Cracks at the surface after 50% cold rolling. Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 13

Figure 8. Deformation twins in one of the 33% cold rolled samples. Martensite grows along the Appl. Sci. 2019, 9, 3265 7 of 12 deformation twin boundaries.

The high levels of cold work also generated small cracks at the surface of the welded samples (Figure9 ).9). This This leads leads to to a decreasea decrease in thein performancethe performance of the of protective the protective layer aslayer its continuity as its continuity is broken is andbroken a slight and reductiona slight reduction of the mechanical of the mechanical properties properties when compared when tocompared a sample to with a sample no cracks. with no cracks.

Figure 9. Cracks at the surface after 50% cold rolling.

The results of hardness tests are shown in Table2, in which the ﬁgures given are the average values obtained from the ﬁve samples of each batch with their standard deviations. As can be seen, a 50% cold work doubles the hardness of the steel, both in the joint and the base metal, which have similar hardness values in all cases.

Table 2. Hardness test results in the welded, heat aﬀected and base material zones.

Welded Zone HAZ Base Material Cold Work (HV) (HV) (HV) 0% 220 5 221 5 217 2.5 ± ± ± 17% 304 1.5 340 2 291 3.7 ± ± ± 33% 379 16 435 12 387 7 ± ± ± 50% 421 3 470 5.7 416 14 ± ± ± 57% 480 3 489 4.5 463 1.3 ± ± ±

3.2. Tensile Tests The results of the tensile tests can be seen in Table3, in which the values for the yield strength and the tensile strength are the average values obtained from the ﬁve samples of each batch, with their standard deviations. Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 13

The results of hardness tests are shown in Table 2, in which the figures given are the average values obtained from the five samples of each batch with their standard deviations. As can be seen, a 50% cold work doubles the hardness of the steel, both in the joint and the base metal, which have similar hardness values in all cases.

Table 2. Hardness test results in the welded, heat affected and base material zones.

Welded Zone∇ HAZ∇ Base Material∇ Cold Work (HV) (HV) (HV) 0% 220 ± 5 221 ± 5 217 ± 2.5 17% 304 ± 1.5 340 ± 2 291 ± 3.7 33% 379 ± 16 435 ± 12 387 ± 7 50% 421 ± 3 470 ± 5.7 416 ± 14 57% 480 ± 3 489 ± 4.5 463 ± 1.3

3.2. Tensile Tests The results of the tensile tests can be seen in Table 3, in which the values for the yield strength and the tensile strength are the average values obtained from the five samples of each batch, with Appl. Sci. 2019, 9, 3265 8 of 12 their standard deviations.

Table 3.3. Resistance characteristics obtained from tensiletensile tests.tests.

ColdCold Work Work Tensile Tensile Strength Strength (MPa)(MPa) Yield Strength Yield Strength (MPa) (MPa) 0%0% 480 480 ± 1717 180 ± 18014 14 ± ± 17%17% 760 760 ± 2020 360 ± 36016 16 ± ± 33% 800 16 580 15 33% 800 ± 16 580 ± 15 ± 50% 1170 25 930 20 ± ± 57%50% 1170 1200 ± 2529 930 ± 1120 20 26 ± ± 57% 1200 ± 29 1120 ± 26 3.3. Corrosion Tests 3.3. Corrosion Tests Figure 10 shows an example of the pitting corrosion tests on a cold rolled sample. Figure 10 shows an example of the pitting corrosion tests on a cold rolled sample.

Figure 10. Pitting at the welding and HAZ zones of a 33% cold work sample. Figure 10. Pitting at the welding and HAZ zones of a 33% cold work sample. Table4 gives the average value of the yield strength and the average number of pits. The number Table 4 gives the average value of the yield strength and the average number of pits. The number of pits was determined in a 20 mm long area of the HAZ (whose limits are equidistant to the weld) of pits was determined in a 20 mm long area of the HAZ (whose limits are equidistant to the weld) using a 50 magnifying glass and the image counting system provided by the software ImageJ. using a 50×× magnifying glass and the image counting system provided by the software ImageJ. Table 4. Yield strength and number of pits per m2. Table 4. Yield strength and number of pits per m2. Cold Work Yield Strength (MPa) Pits Number (103/m2) Pits Number∇ 0%Cold Work Yield 180 Strength (MPa) 175 (103/m2) 17% 360 185 33% 580 225 50% 930 320 57% 1120 400

Except for the undeformed samples, all of them presented big pits that perforated the samples.

4. Analysis of Results and Discussion Figure 11 shows the correlation between the cold work level (percentage of width reduction) and the number of pits (an indicator of the corrosion degree). The following expression, obtained by a least squares ﬁt, models this correlation:

0.0585 x y = 165316 + 8343 e · , · where:

y = Pits number/m2 x = Cold work (%) Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13

0% 180 175 17% 360 185 33% 580 225 50% 930 320 57% 1120 400

Except for the undeformed samples, all of them presented big pits that perforated the samples.

= 165316 + 8343 · .·, where:

y = Pits number/m2 x = Cold work (%) According to [21], the linear dependence of the pit generation rate on the potential suggests that the pitting process is controlled by an electromechanical breakdown of the passive film. This Appl.conclusion Sci. 2019 ,could9, 3265 support the correlation found between the cold work level and the number of9 ofpits 12 at the present study, that has been represented in Figure 11.

500 450 400 ) 2

m 350 3/ 300 250 200 150 Number of pits (x10 100 50 0 0 102030405060 Cold work (%)

FigureFigure 11.11. Graphical representation ofof thethe correlationcorrelation betweenbetween thethe coldcold workwork andand numbernumber ofof pitspits inin aa 10%10% massmass solutionsolution ofof FeClFeCl3·6H6H 2O. 3· 2 AccordingAbout the result to [21 ],of thethe linearcorrosion dependence test, we must of the first pit consider generation that ratethe effectiveness on the potential of the suggests surface thatlayer the of chromium pitting process oxide, is which controlled acts as by a an protective electromechanical agent against breakdown corrosion of inthe stainless passive steels, film. can This be conclusionreduced due could to the support rolling the mechanical correlation effects found that between could lead the cold to a workthickness level reduction and the number and/or breakage of pits at theof this present protective study, thatlayer, has as been evidenced represented by the in Figurepresence 11. of small surface cracks in the cold rolled About the result of the corrosion test, we must first consider that the effectiveness of the surface layer of chromium oxide, which acts as a protective agent against corrosion in stainless steels, can be reduced due to the rolling mechanical effects that could lead to a thickness reduction and/or breakage of this protective layer, as evidenced by the presence of small surface cracks in the cold rolled samples. Furthermore, the study carried out in [33] focused on the welding of 304 L austenitic stainless steel by the shielded metal arc welding process using a standard 308 L electrode, shows that, during sensitization, chromium in the matrix precipitates out as carbides and intermetallic compounds on grain boundary, decreasing the corrosion resistance. Similar results are found in [34], and [35] which associates the precipitation of chromium nitride with the reduction of the corrosion resistance of the nitrided layer. In the AISI 304 stainless steel welding, pitting is concentrated in Cr-depleted regions adjacent to the carbide precipitates [11]. The studies about the effect of microstructural changes in the AISI 304 stainless steel induced by TIG welding, and surely other welding techniques, on the pitting corrosion behavior conclude that the process made the weld metal and HAZ zone more sensitive to the pitting corrosion than the base metal [36]. This work also concludes that high heat-input (and low cooling rate) was likely to induce the segregation of alloying elements and formation of Cr-depleted zones, resulting in decreasing the corrosion resistance [36]. Even more, under [11] study, TIG-welds exhibit lower pitting corrosion resistance than friction and electron beam welds. In fact, austenitic stainless steels are prone to sensitization when subjected to higher temperatures during the manufacturing process. In particular, the study of [27] emphasizes the essential role of temperature-affected variations of the properties of oxide films and their effect on the pitting corrosion in the AISI 304 stainless steel. Appl. Sci. 2019, 9, 3265 10 of 12

The pitting corrosion occurs as a result of a breakdown of the protective passive film on the metal surface [22,37–39] and according to [25], the lowest pitting corrosion resistance behavior is shown in the heat affected zone. A relevant experimental result, described by [40], is the appearance of the strain-induced martensite, transformed from austenite during the cold working process that can be observed in Figure4. Which could be related with the appearance of corrosion points, since the formation of micro-pitting occurs preferentially in areas where the tensile residual stresses are the highest [41]. After analyzing the reduction of the anti-corrosion protection in AISI 304 rolled stainless steel welding, it is worth considering how to mitigate the factors that trigger it (chromium concentration decreases in the protective layer and deformation stresses in the material). We have previously commented that the use of a low-carbon filler material reduces chromium carbide formation in the grain boundaries during welding and so, maintains chromium concentration at adequate levels [6,7]. Regarding the deformation stresses reduction, heat treatments are not feasible. As a result of the plate thicknesses considered, stress-reducing heat treatments will cause serious deformations such as unacceptable flatness errors. Another consideration pointed out by [42] is to improve the corrosion resistance of austenitic stainless steels, like AISI 304, through surface finishing treatments, which has been demonstrated beneficial regarding the localized pitting corrosion properties in stainless steels [43]. However, it also must be considered that the residual stress introduced by certain surface finishes affects the corrosion behavior of austenitic stainless steel [13]. Finally, due to the practical impossibility of avoiding the negative effects on corrosion resistance of the manufacture of thin AISI 304 rolled stainless steel welded plates, one alternative would be to select the thickness of the material used according to its expected use conditions, for which the information in Table4 could be used.

5. Conclusions Corrosion resistance of AISI 304 rolled stainless steel welding can be altered at both the welded and the HAZ zones due to the effects of cold rolling and the metallurgic transformations and physic-chemical processes that occur during welding, including residual stresses distribution. The increment of the pitting corrosion in AISI 304 rolled stainless steel welding can be attributed to a combination of factors: The reduction of the anti-corrosion protection, the microstructural changes resulting from the cold rolling process and the internal increased stresses. Both the chromium concentration decrease, that causes thickness reduction and/or breakage of the chrome oxide protective layer, and the increment of stresses in the samples have been observed in the experimental results. Martensite induced by deformation, transformed from austenite during rolling has also been observed in the experimental results. All these changes, which promote the local pitting corrosion in the AISI 304 rolled stainless steel weld, are difficult to prevent. The pitting corrosion degree in AISI 304 cold rolled welds was evaluated in a FeCl3 solution following the ASTM G48-11 standard. The corrosion resistance of the samples was measured using the number of pits and correlated with the deformation level of these samples. The number of pits varied from 175 103 pits/m2 for the undeformed samples to 400 103 pits/m2 for a 57% cold work level. · · The exponential expression that correlates cold work levels and pits number, if confirmed, can be useful in the design and material selection for products and facilities including the AISI 304 rolled stainless steel butt-welded, since it provides a simple (not expensive) method to assess the material thickness suitability regarding its corrosion resistance.

Author Contributions: In this investigation, F.-J.C.-C., M.P.-G. and F.S.V. conceived and designed the experiments; F.-J.C.-C., L.S.G. and M.-A.P.-P. performed the experiments; M.P.-G., L.S.G. and F.S.V. analyzed the data; M.-A.P.-P. contributed materials/analysis tools; M.P.-G., L.S.G. and F.S.V. wrote the paper. Funding: This research received no external funding. Appl. Sci. 2019, 9, 3265 11 of 12

Acknowledgments: The authors deeply thank the Universitat Politècnica de València (Spain), for the support of this research. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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