Effect of Aging Temperature on Corrosion Behaviorof Sintered 17-4 PH Stainless Steel in Dilute Sulfuric

Acid SolutionAneta Szewczyk-Nykiel and Jan Kazior

(Submitted October 29, 2015; in revised form April 12, 2017; published online June 15, 2017)

The general corrosion behavior of sintered 17-4 PH stainless steel processed under different processingconditions in dilute sulfuric acid solution at 25 �C was studied by open-circuit potential measurement andpotentiodynamic polarization technique. The corrosion resistance was evaluated based on electrochemicalparameters, such as polarization resistance, corrosion potential, corrosion current density as well as cor-rosion rate. The results showed that the precipitation-hardening treatment could significantly improve thecorrosion resistance of the sintered 17-4 PH stainless steel in studied environment. As far as the influence ofaging temperature on corrosion behavior of the sintered 17-4 PH stainless steel is concerned, polarizationresistance and corrosion rate are reduced with increasing aging temperature from 480 up to 500 �Cregardless of the temperature of solution treatment. It can be concluded that the highest corrosion resis-tance in 0.5 M H2SO4 solution exhibits 17-4 PH after solution treatment at 1040 �C followed by aging at480 �C.

Keywords corrosion resistance, hardness, 17-4 PH stainless steel

1. Introduction

AISI 316L and 17-4 PH (UNS S17400, AISI 630) are twogrades of wrought stainless steels commonly used in corrosiveenvironments (Ref 1-8). The results of corrosion resistance testin sulfuric acid (tested per ASTM G 31-72, 100C, 24 hours)showed that the corrosion rate of wrought AISI 316L was morethan nine times better than 17-4 PH stainless steel (Ref 9). 17-4PH is a precipitation-hardening martensitic stainless steelcontaining approximately 3-5 wt.% copper (Ref 10-15). Thissteel exhibits high mechanical properties with good corrosionresistance (Ref 14-16) and is widely used in aerospace,chemical, petrochemical and food industries (Ref 10-15). Afterthe solution treatment at 1040 �C, 17-4 PH stainless steel has asoft martensitic structure supersaturated with Cu (Ref 10, 11).The subsequent aging treatment results in precipitation hard-ening of 17-4 PH due to the formation of a submicroscopic,copper-rich phase (Ref 11). Aging temperatures usually varyfrom 480 to 620 �C (Ref 10, 11, 14).

The previous studies on wrought 17-4 PH stainless steelswere mainly focused on an analysis of chemical composition,microstructure, mechanical properties and also corrosion resis-tance (Ref 8-10, 13, 14, 17-21). The corrosion resistance ofprecipitation-hardening steels depends on their chemical com-positions as well as microstructure. And hence heat treatment

effects on the corrosion behavior. It turns out that aging cancause certain loss of corrosion resistance of PH steels (Ref 5).

For example, it was found that the pitting corrosionresistance of 17-4 PH can be significantly improved by usingthe high-temperature (420 �C as well as 500 �C) plasmanitriding (Ref 16, 21).

Furthermore, there are several studies revealing the effect ofaging temperature on the microstructure, mechanical properties,corrosion resistance in chloride solutions and wear resistance of17-4 PH stainless steel (Ref 7, 11-13, 15, 16, 19, 22-25).

Considering the corrosion behavior of 17-4 PH stainlesssteel, most of the published articles are based on results fromsimple immersion tests in solutions. There are some works inwhich the corrosion resistance of these materials was evaluatedby electrochemical techniques (Ref 7, 13, 16, 25).

It was shown that wrought 17-4 PH stainless steel has a highresistance to stress corrosion cracking. While age-hardeningtreatment increases its sensitivity to stress corrosion cracking(Ref 7), this behavior is a consequence of the compositions ofphases (17-4 PH consists of a mixture of martensite andmutable content of d-ferrite and e-copper precipitation depend-ing on the aging conditions), this steel is susceptible to pittingcorrosion in the chloride-containing environment (Ref 19).Potentiodynamic polarization measurements (by utilizing aslow scan rate of 0.05 mV/s) indicated that by increasing agingtemperature from 480 to 550 �C, the pitting potential of 17-4PH steel is considerably increased, but further rising the agingtemperature up to 620 �C reduces the pitting potential. It isbecause of differences in volume fraction of ferrite, morphol-ogy and distribution of copper-rich precipitates and the amountof reverted austenite in steel aged at 620 �C (Ref 25).

According to Raja and Prasad Rao Ref 18, general corrosionresistance of 17-4 PH steel was not affected by the thermaltreatments significantly. However, solution annealing (1050 �C,30 min, air cooling) followed by aging (480 �C, 1 h, aircooling) resulted in uniform and relatively better corrosionresistance.

Aneta Szewczyk-Nykiel and Jan Kazior, Institute of MaterialEngineering, Cracow University of Technology, Al. Jana Pawła II37, 31-864 Krakow, Poland. Contact e-mails: aneta.szewczyk-nykiel@mech.pk.edu.pl and jan.kazior@mech.pk.edu.pl.

JMEPEG (2017) 26:3450–3456 �The Author(s). This article is an open access publicationDOI: 10.1007/s11665-017-2778-4 1059-9495/$19.00

3450—Volume 26(7) July 2017 Journal of Materials Engineering and Performance

In the case of stainless steels, intergranular corrosion iscaused by formation of chromium carbides, which are mainlyconcentrated in the grain boundaries. This is why, chromiumdepletion occurs and regions around the grain boundariesbecome anodic. In effect, deterioration of corrosion resistanceis observed. After solution treatment, 17-4 PH steel showed avery low degree of sensitization. Similarly, this steel exhibitedvery low degrees of sensitization after aging at 480 �C for 4 has well as 2 h. When the aging was carried out at temperaturehigher than 495 �C, 17-4 PH showed high degrees ofsensitization. This indicates an important increase in intergran-ular corrosion susceptibility (Ref 10).

Not much published information is available on corrosionresistance of 17-4 PH steel in dilute sulfuric acid (Ref 5, 10,12). However, the effect of aging temperature on corrosionresistance of 17-4 PH steels in dilute sulfuric acid can be found.Namely, the corrosion rates in dilute sulfuric acid after differentheat treatments on the bar specimens of wrought 17-4 PH steelhave been designated (Ref 5). It was found that the corrosionrate at the higher aging temperature (around 610 �C) wasgreater than that at the lower aging temperature (around460 �C). Also the damage of the specimen was also stronger.

In the case of wrought 17-4 PH stainless steel aged near460 �C (heat treatment at 1050 �C), the hardness reached thehighest value as well as the corrosion resistance was the mostexcellent because of the best precipitation strengthening effectof fine and dispersed e-Cu phase in a single martensite matrix(Ref 12).

Other results (Ref 12) also indicate that wrought 17-4 PHsteel aged at 460 �C showed the best erosion-corrosionresistance. In this case when aging temperature ranged from400 to 610 �C, the corrosion rate firstly decreased and reachedthe lowest value near the aging temperature of 460 �C. Then,the corrosion rate started to increase when the aging temper-ature continued to increase. Generally, pure corrosion rate wasvery small.

And finally it can be stated that the corrosion resistance ofwrought 17-4 PH steel depends strongly on heat treatment. Inthe case of high-strength sintered alloys, such as 17-4 PH,achievement of a full or near full density is essential in order toachieve the full benefit of their superior mechanical properties.The feasibility of using the conventional PM process to produce17-4 PH with sintered density greater than 7.3 g/cm3 has beendemonstrated by Reinshagen and Witsberger Ref 26. However,still there is lack of the corrosion data for sintered precipitation-hardening 17-4 PH stainless steel.

For sintered 17-4 PH stainless steel corrosion data should beconsidered somewhat otherwise from those of wrought and caststainless steels. On the one hand, this is due to the lack ofcorrosion resistance standard for sintered stainless steels; on theother hand, it arises from the larger effective surface areas ofsintered stainless steels and chemical reactions taking placeduring atomization and sintering. Furthermore, the flexibility ofPM processing, including opportunities with alloying andsurface modifications, should close any existing gaps and resultin the development of superior materials, as has been in thecase in other material groups.

The aim of this paper is to estimate the corrosion behavior ofsintered and heat-treated 17-4 PH stainless steels. In the presentstudy, the influence of aging temperature on corrosion resis-tance of 17-4 PH stainless steel in dilute sulfuric acid solutionhas been investigated by utilizing open-circuit potential mea-surement and potentiodynamic polarization technique.

2. Experimental Procedure

A water-atomized powder of 17-4 PH martensitic precipi-tation-hardening stainless steel (corresponding with standards:ASTM-A564 grade 630; UNS S17400) supplied by AMETEKwas used. It should be noted that the investigated powdercontained addition of lubricant in form of Acrawax in quantityof 0.75 wt.% chemical composition of 17-4 PH stainless steelpowder (wt.%) is the following: 16.28 Cr, 4.28 Ni, 4.04 Cu,0.73 Si and 0.32 Nb. Apparent density of this powder was2.54 (g/cm2), while its flow was 31 (s/50 g). Typical powderparticle size was below 150 lm.

The 17-4 PH steel powder was pressed in rigid die under600 MPa into cylindrical specimens (20 mm of diameter and5 mm of height). Then, samples were sintered at 1340 �C inhydrogen atmosphere. The sintering process was carried out ina laboratory Nabertherm P330 furnace. The time for isothermalsintering was 30 min. A heating rate to reach the sinteringtemperature as well as cooling rate from sintering temperaturewas 10 �C/min. In order to remove the lubricant, the sampleswere held at a temperature of 400 �C for 60 min duringheating. To compare results of investigation, some as-sinteredsamples were left (designation A). The remaining sinteredspecimens were solution treated at 1020 and 1040 �C for30 min and oil quenched. And then they were aged at threetemperatures, namely 480, 490 and 500 �C, for 1 h. Descrip-tion of the samples designation is shown in Table 1. It refers topreparation conditions of samples used in studies.

The density and porosity of investigated steels weremeasured by the water displacement method (according todemands of PN-EN ISO 2738:2001 norm). The hardness(HV10) was determined.

Corrosion characteristics are commonly used and consideredas one of the important research methods for evaluatingcorrosion behavior of materials in aggressive environments. Inorder to estimate the corrosion behavior of both as-sintered andaged 17-4 PH stainless steels, electrochemical study wasperformed. The potentiodynamic tests were conducted usingATLAS 0531 UI&IA potentiostat, featuring a conventionalthree-electrode electrochemical cell. This cell consisted ofworking, reference and counter electrodes. The specimen wasthe working electrode. The area exposed was 1.33 cm2.Saturated calomel electrode (SCE) was chosen as the referenceelectrode. Platinum electrode was used as the counter electrode.All experiments were carried out in 0.5 M H2SO4 solution atroom temperature. This solution was prepared from analyticalgrade 97%ÆH2SO4 and distilled water. All samples weregrinded, then degreased with acetone, washed with distilledwater and dried prior to tests. Open-circuit potential (OCP)measurement and potentiodynamic polarization test wereperformed. OCP is often used as a criterion for the corrosionbehavior of materials in a corrosive medium. The open-circuit

Table 1 Description of the samples designation

Temperature of solution treatment, �C

Temperature ofaging, �C

480 490 500

1020 B C D1040 E F G

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potential (OCP) of investigated steels was investigated during2 h of immersion in solution. The potential of the samples wasrecorded as a function of time. After the specimens reached astable value of open-circuit potential, the potentiodynamicmeasurements were made. The potential of the electrode wasswept at a rate of 1 mV/s from an initial potential of �250 mVversus the OCP to a final potential of 1500 mV. Based on theanodic potentiodynamic polarization curves, corrosion param-eters were determined. Corrosion potential (Ecorr) and corrosioncurrent density (Icorr) were evaluated by Tafel extrapolationmethod. The polarization resistance (Rpol) was determinedusing linear polarization method (called Stern method) andTafel extrapolate method (called Stern–Geary method). Thecorrosion rate in (mm/y) was calculated from the corrosioncurrent density (Ref 27).

Metallographic cross sections were prepared using standardprocedures. Namely, the samples were cross sectioned,mounted, ground, polished and etched (aqua regia reagent).The microstructural study was done with Nikon Eclipse ME600P light optical microscope and scanning electron micro-scopy (SEM). Besides, EDS analysis was carried out in order todetermine the chemical composition of studied materials beforeand after corrosion test. Mainly, SEM and EDS were used tostudy the surface state of working electrode.

3. Results and Discussion

The results of density and porosity measurements show thatafter sintering process 17-4 PH stainless steel has good densityand low porosity. The density of sintered 17-4 PH stainlesssteel was 7.46 g/cm3, which means that the relative densityreached quite high value (almost 98%). The open and totalporosity was about 0.6 and 2.3%, respectively. As far as theeffect on heat treatment on physical properties is concerned, itcan be observed that aging temperature does not influence thedensity and porosity of 17-4 PH steel. Regardless of theconditions of solution treatment followed by aging, thedifference in values of both density and porosity is in thesecond place after the decimal point.

The examples of the microstructure for sintered as well aged17-4 PH stainless steel are presented in Fig. 1 and 2.

The heat treatment did not cause a distinct change in themorphology of porosity of 17-4 PH stainless steel. It can beobserved that the shape of pores is similar to spheroidal. Butafter heat treatment, the pores are a little smaller and the amountof them is higher. The microstructure of sintered 17-4 PHstainless steel consists of the martensite matrix and d-ferrite.The microhardness of martensite in as-sintered 17-4 PHstainless steel is equal to about 294 HV0.01. In the case ofas-aged 17-4 PH stainless steel, the microhardness of marten-site is higher, and furthermore, the higher aging temperaturefrom 480 to 500 �C and the lower microhardness from about445 to 395 HV0.01, respectively, are obtained.

The effect of aging temperature on the hardness of studiedsteel was investigated. Figure 3 shows the hardness of 17-4 PHstainless steel as a function of the heat treatment conditions. Itshould be pointed that the hardness of as-sintered 17-4 PHstainless steel (specimen A) is equal to 240 HV10. As it mightbe expected the values of hardness of aged 17-4 PH stainlesssteels are higher when compared with hardness value ofsintered steel. The increase in hardness of the steel can beascribed by microstructural changes, due to precipitationstrengthening effect of fine and dispersed e-Cu phase inmartensite matrix [as in the case of wrought 17-4 PH stainlesssteel (Ref 5, 12)]. Furthermore, when aging temperatureincreases hardness of investigated steels decreases for bothsolution treatment temperatures. Nevertheless the higher tem-perature of solution treatment, the higher hardness of aged steelwas obtained.

It is well known that the corrosion resistance of steels suchas 17-4 PH is associated with their ability to passivation. Thepassivation means an increase in resistance to corrosion byoxidation and formation of protective film on the surface.

The open-circuit potential as a corrosion parameter indicatesthe thermodynamical tendency of a material to electrochemicaloxidation in a corrosive medium. It is well known that theopen-circuit potential may vary with time. This is because ofchanges in the nature of the surface of the electrode. Sooxidation, formation of the passive layer or resistance mayoccur.

The variations in open-circuit potential for all investigatedsteels immersed in 0.5 M H2SO4 solution were monitored.Some obtained results are presented in Fig 4 and 5. The values

Fig. 1 Microstructure of sintered 17-4 PH stainless steel Fig. 2 Microstructure of heat-treated 17-4 PH stainless steel (solu-tion treatment at 1040 �C, aging at 480 �C

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of open-circuit potential at the beginning of the test and after2 h of immersion in 0.5 M H2SO4 solution were determined forall the samples.

In the case of as-sintered 17-4 PH stainless steel, thevariation of open-circuit potential with time is different thanthat for heat-treated 17-4 PH stainless steel. It should be notedthat as-sintered 17-4 PH stainless steel has the lowest OCP atthe beginning of test and potential shows the tendency to shifttoward more negative values. It seems that the intensity ofcorrosion at as-sintered 17-4 PH stainless steel is rathersubstantial. It can be confirmed by the fact of constant change(reduction) of potential value during the test. Afterward, theOCP indicates slower changes with time (the reduction inpotential decreases). The variation of open-circuit potentialwith time is similar for steel after all performed heat treatments.At the beginning of test, it can be seen sudden OCPdisplacement toward positive values (Fig. 5). Then, the poten-tial decreases slowly and after some period of time it stabilizesaround a stationary value. The initial increase in OCP suggeststhat the formation of the oxide film on the sample surfaceoccurs. This kind of behavior improves corrosion resistance.The constant value of potential suggests the thermodynamicalstability of the passive layer and resistance to chemicaldissolution in solution.

From Fig. 4, some conclusions can be obtained. The higheraging temperature, the higher open-circuit potential. Regardlessof conditions of solution treatment, 17-4 PH stainless steel agedat 480 �C reaches the highest values of OCP. As far ascorrosion behavior is concerned, sintered 17-4 PH stainless

steel behaves worse than the same steel after solution treatmentfollowed by aging. It can be stated that heat treatment improvescorrosion resistance of investigated 17-4 PH stainless steel.

Potentiodynamic polarization study in 0.5 M H2SO4 solu-tion was performed in order to obtain an information of thecorrosion behavior of the sintered 17-4 PH stainless steel afterheat treatment under different conditions. Figure 6 shows theregistered potentiodynamic curve for the sintered as well asaged at different temperatures 17-4 PH stainless steel.

As can be seen (Fig. 6), the polarization curve for as-sintered 17-4 PH stainless steel is different from curvesregistered for the same steel after heat treatment. Although,comparison of the registered curves reveals that cathodic Tafellines are nearly identical for all investigated materials. How-ever, the same observation cannot be made with respect to theanodic Tafel lines. In the case of as-sintered 17-4 PH stainlesssteel, potentiodynamic polarization studies demonstrate that: aclearly visible active-passive peak does not appear, a widepassive range occurs and the values of anodic current densityare significantly lower.

It can be noted that the curves registered for heat-treated 17-4 PH stainless steel are in fact almost identical. It can beobserved active-passive transition maximum. A region ofactivation, passivation and transpassivation can be separatedin polarization curves. Moreover, two stages of passive processcan be observed. Initially, low-potential passive region and thenhigh-potential passive region appear. After this, the increase incurrent density occurs. It is related to onset of transpassivedissolution.

The parameters such as corrosion potential (Ecorr), corrosioncurrent density (Icorr), polarization resistance (Rpol) evaluatedbased on polarization curves as well as corrosion rate have beencalculated for all investigated materials. The Rpol was deter-mined using Stern method as well as Stern–Geary method. Theelectrochemical data are summarized in Table 2.

As expected, the conditions of sample preparation affectedthe parameters such as potential corrosion and corrosion currentdensity. The Ecorr of as-sintered 17-4 PH stainless steel islocated at �0.388 (V versus SCE). While, the value ofcorrosion current density amounts to 2.36E-04 (A/cm2) and isthe lowest as compared to the values of Icorr assessed for steelafter heat treatment. In the case of heat-treated 17-4 PHstainless steel, the corrosion potential is shifted toward to morepositive values and the corrosion current density becomessmaller with decreasing aging temperature from 500 up to480 �C. The lowest value of corrosion current density wasobtained for 17-4 PH stainless steel after solution treatment at1040 �C and aging at 480 �C.

Fig. 3 Effect of aging temperature on hardness of 17-4 PH stain-less steel

Fig. 4 Open-circuit potential variation with time for investigated17-4 PH stainless steel

Fig. 5 Variation of open-circuit potential with time for the first30 min for 17-4 PH stainless steel after solution treatment at1040 �C followed by aging at 480 �C

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The results in Table 2 indicate that the polarization resis-tance of 17-4 PH stainless steel (regardless of the used methodof determination) changes as a function of the heat treatmentconditions. It can be noticed that Rpol is reduced with increasingaging temperature from 480 up to 500 �C. This tendency isobserved regardless of the temperature of solution treatment.Comparison of the values of polarization resistance revealedthat the highest resistance to corrosion was obtained for as-sintered 17-4 PH stainless steel after solution treatment at1040 �C followed by aging at 480 �C.

Obtained results also demonstrate that solution treatment at1040 �C followed by aging at 480 �C resulted in the lowestcorrosion rate. On the other hand, un-treated 17-4 PH stainlesssteel reached the highest corrosion rate. It can be observed thatthe corrosion rate tends to increase when aging temperatureincreases.

Figures 7 and 8 show SEM images of surface of as-sintered17-4 PH stainless steel before and after its immersion in 0.5 MH2SO4 solution. EDS analyses were performed in order todistinguish the differences in chemical composition in thespecimens. The microanalysis of chemical composition wasperformed at different regions, and results are shown inTables 3 and 4.

In the case of as-sintered 17-4 PH stainless steel, areanumbered 1 was designated in the martensite matrix, while area

numbered 2 was d-ferrite. Microanalysis of chemical compo-sition (Table 3) indicates that the main elements are Fe, Cr, Niand Cu in both areas. In the case of sintered 17-4 PH stainlesssteel after its immersion in 0.5 M H2SO4 solution, the analysis

Fig. 6 Potentiodynamic polarization curves of 17-4 PH stainless steel in 0.5 M H2SO4 solution

Table 2 Values of corrosion parameters for the investigated 17-4 PH steel

Designation

Corrosion parameters

Ecorr (V versu SCE) Icorr, A/cm2

Rpol, X cm2

Corrosion rate, mm/yStern method Stern–Geary method

A �0.388 2.36E�04 66.46 75.07 2.631B �0.357 1.19E�04 120.11 120.86 1.326C �0.375 2.04E�04 93.44 93.91 2.274D �0.388 2.10E�04 50.27 77.67 2.341E �0.385 6.59E�05 122.50 188.30 0.735F �0.388 7.33E�05 105.89 149.65 0.817H �0.393 1.61E�04 62.46 65.10 1.717

Fig. 7 SEM microstructure of surface of sintered 17-4 PH stainlesssteel before its immersion in 0.5 M H2SO4 solution

3454—Volume 26(7) July 2017 Journal of Materials Engineering and Performance

was performed at points 1, 2 and 3. Microanalysis of chemicalcomposition (Table 4) indicated that the main elements in pointnumber 3 are Fe, Cr, Ni and Cu. The contents of these elementsare almost the same as in the chemical composition of 17-4 PHsteel. The main elements in point number 1 and 2 are O, Fe, Sand Cr. It should be noted that the contents of elements such asFe, Cr, Ni and Cu in point 1 as well as 2 are lower than in point3.

Finally, in the case of sintered stainless steels processed viaconventional powder metallurgy routes the corrosion resistancecan be improved significantly with increasing density. Theundesired effect of pores is assigned to large internal surfaceareas of sintered parts, which, at the typical structural parts, arestill two orders of magnitude larger than their exteriorgeometric surface areas and therefore can be subject toincreased general corrosion. Furthermore, the pores attributedto a lack of passivation within the pores of a sintered part.Therefore, the corrosion in an acid environment can beconsidered as the action of a hydrogen concentration cell

between the external surface of a part and its internal poresurface. The surface of the pores acts as the anode and theengineering surface as the cathode. Metal dissolution occursprimarily in the interior of the material, and after the increasingtime, the activation process comes to an end and the potentialincreases.

Based on the obtained results, the following electrochemicalreactions can be proposed in the case of uniform corrosion insulfuric acid. The electrochemical reaction at the surfaceincludes the anodic dissolution of iron according to thefollowing equations:

oxidation of iron to ferrous ionsð Þ Fe ! Fe2þ þ 2e� ðEq 1Þ

oxidation of ferrous ions to form ferric ionsð Þ Fe2þ !Fe3þþ e�

ðEq 2Þ

and the formation of a passive film

3H2Oþ 2Fe3þ ! Fe2O3 þ 6Hþ ðEq 3Þ

While, the cathodic reactions are the hydrogen evolution reac-tion

2 Hþ þ 2e� ! H2 ðEq 4Þ

and the direct reduction in sulfuric acid

H2SO4 ! Hþ þ HSO�4 ðEq 5Þ

HSO�4 ! Hþ þ SO2�

4 ðEq 6Þ

The aqueous electrolytic solution contains ferrous and ferricions in sulfuric acid. The dissociation of sulfuric acid causesthe formation of the following ions: H+, Fe2+, Fe3+, SO4

2�

and HSO4�.

Formation of a ferrous sulfate salt:

4H2Oþ Fe2þ þ SO2�4 ! FeSO4 � 4H2O ðEq 7Þ

In conclusion, the formation of a ferrous sulfate salt filmplays a role in the passivation of iron alloys in sulfuric acid.This mechanism was also suggested by other authors (Ref 28,29).

Fig. 8 SEM microstructure of surface of sintered 17-4 PH stainlesssteel after its immersion in 0.5 M H2SO4 solution

Table 3 Results of EDS analysis of surface of sintered 17-4 PH stainless steel before its immersion in 0.5 M H2SO4

solution

Point number

Chemical composition, wt.%

Cr Fe Ni Cu

1 22.589 74.237 1.737 1.4372 16.358 74.710 4.592 4.341

Table 4 Results of EDS analysis of surface of sintered 17-4 PH stainless steel after its immersion in 0.5 M H2SO4

solution

Point number

Chemical composition, wt.%

Cr Fe Ni Cu O S

1 5.259 29.524 1.466 1.278 45.582 16.8912 3.688 29.603 1.299 0.766 46.844 17.8003 17.492 74.243 4.264 4.001 … …

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4. Conclusion

The general corrosion behavior of sintered 17-4 PH stainlesssteel processed under different heat treatment conditions in0.5 M H2SO4 solution at ambient temperature was studied byopen-circuit potential measurement and potentiodynamic polar-ization technique. In particular, the influence of aging temper-ature on corrosion resistance of sintered 17-4 PH steel has beenstudied.

The corrosion resistance was evaluated based on corrosionparameters, such as potential corrosion, corrosion currentdensity polarization resistance as well as corrosion rate. Theresults showed that the precipitation-hardening treatment can beimproved significantly the corrosion resistance of the sintered17-4 PH steel in diluted sulfuric acid. As far as the influence ofaging temperature on corrosion behavior of the sintered 17-4PH steel is concerned, it can be concluded that the highestcorrosion resistance in 0.5 M H2SO4 solution exhibits 17-4 PHafter solution treatment at 1040 �C followed by aging at480 �C. It can be concluded that formation of a ferrous sulfatesalt plays a role in the passivation of iron alloys in sulfuric acid.

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3456—Volume 26(7) July 2017 Journal of Materials Engineering and Performance