Introduction

AISI 316L and 17-4 PH (UNS S17400, AISI 630) are two grades of wrought stainless steels commonly used in corrosive environments (Ref 1-8). The results of corrosion resistance test in sulfuric acid (tested per ASTM G 31-72, 100C, 24 hours) showed that the corrosion rate of wrought AISI 316L was more than nine times better than 17-4 PH stainless steel (Ref 9). 17-4 PH is a precipitation-hardening martensitic stainless steel containing approximately 3-5 wt.% copper (Ref 10-15). This steel exhibits high mechanical properties with good corrosion resistance (Ref 14-16) and is widely used in aerospace, chemical, petrochemical and food industries (Ref 10-15). After the solution treatment at 1040 °C, 17-4 PH stainless steel has a soft martensitic structure supersaturated with Cu (Ref 10, 11). The subsequent aging treatment results in precipitation hardening of 17-4 PH due to the formation of a submicroscopic, copper-rich phase (Ref 11). Aging temperatures usually vary from 480 to 620 °C (Ref 10, 11, 14).

The previous studies on wrought 17-4 PH stainless steels were mainly focused on an analysis of chemical composition, microstructure, mechanical properties and also corrosion resistance (Ref 8-10, 13, 14, 17-21). The corrosion resistance of precipitation-hardening steels depends on their chemical compositions as well as microstructure. And hence heat treatment effects on the corrosion behavior. It turns out that aging can cause certain loss of corrosion resistance of PH steels (Ref 5).

For example, it was found that the pitting corrosion resistance of 17-4 PH can be significantly improved by using the high-temperature (420 °C as well as 500 °C) plasma nitriding (Ref 16, 21).

Furthermore, there are several studies revealing the effect of aging temperature on the microstructure, mechanical properties, corrosion resistance in chloride solutions and wear resistance of 17-4 PH stainless steel (Ref 7, 11-13, 15, 16, 19, 22-25).

Considering the corrosion behavior of 17-4 PH stainless steel, most of the published articles are based on results from simple immersion tests in solutions. There are some works in which the corrosion resistance of these materials was evaluated by electrochemical techniques (Ref 7, 13, 16, 25).

It was shown that wrought 17-4 PH stainless steel has a high resistance to stress corrosion cracking. While age-hardening treatment increases its sensitivity to stress corrosion cracking (Ref 7), this behavior is a consequence of the compositions of phases (17-4 PH consists of a mixture of martensite and mutable content of δ-ferrite and ε-copper precipitation depending on the aging conditions), this steel is susceptible to pitting corrosion in the chloride-containing environment (Ref 19). Potentiodynamic polarization measurements (by utilizing a slow scan rate of 0.05 mV/s) indicated that by increasing aging temperature from 480 to 550 °C, the pitting potential of 17-4 PH steel is considerably increased, but further rising the aging temperature up to 620 °C reduces the pitting potential. It is because of differences in volume fraction of ferrite, morphology and distribution of copper-rich precipitates and the amount of reverted austenite in steel aged at 620 °C (Ref 25).

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

In the case of stainless steels, intergranular corrosion is caused by formation of chromium carbides, which are mainly concentrated in the grain boundaries. This is why, chromium depletion occurs and regions around the grain boundaries become anodic. In effect, deterioration of corrosion resistance is observed. After solution treatment, 17-4 PH steel showed a very low degree of sensitization. Similarly, this steel exhibited very low degrees of sensitization after aging at 480 °C for 4 h as well as 2 h. When the aging was carried out at temperature higher than 495 °C, 17-4 PH showed high degrees of sensitization. This indicates an important increase in intergranular corrosion susceptibility (Ref 10).

Not much published information is available on corrosion resistance of 17-4 PH steel in dilute sulfuric acid (Ref 5, 10, 12). However, the effect of aging temperature on corrosion resistance of 17-4 PH steels in dilute sulfuric acid can be found. Namely, the corrosion rates in dilute sulfuric acid after different heat treatments on the bar specimens of wrought 17-4 PH steel have been designated (Ref 5). It was found that the corrosion rate at the higher aging temperature (around 610 °C) was greater than that at the lower aging temperature (around 460 °C). Also the damage of the specimen was also stronger.

In the case of wrought 17-4 PH stainless steel aged near 460 °C (heat treatment at 1050 °C), the hardness reached the highest value as well as the corrosion resistance was the most excellent because of the best precipitation strengthening effect of fine and dispersed ε-Cu phase in a single martensite matrix (Ref 12).

Other results (Ref 12) also indicate that wrought 17-4 PH steel aged at 460 °C showed the best erosion-corrosion resistance. In this case when aging temperature ranged from 400 to 610 °C, the corrosion rate firstly decreased and reached the lowest value near the aging temperature of 460 °C. Then, the corrosion rate started to increase when the aging temperature continued to increase. Generally, pure corrosion rate was very small.

And finally it can be stated that the corrosion resistance of wrought 17-4 PH steel depends strongly on heat treatment. In the case of high-strength sintered alloys, such as 17-4 PH, achievement of a full or near full density is essential in order to achieve the full benefit of their superior mechanical properties. The feasibility of using the conventional PM process to produce 17-4 PH with sintered density greater than 7.3 g/cm3 has been demonstrated 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 be considered somewhat otherwise from those of wrought and cast stainless steels. On the one hand, this is due to the lack of corrosion resistance standard for sintered stainless steels; on the other hand, it arises from the larger effective surface areas of sintered stainless steels and chemical reactions taking place during atomization and sintering. Furthermore, the flexibility of PM processing, including opportunities with alloying and surface modifications, should close any existing gaps and result in the development of superior materials, as has been in the case in other material groups.

The aim of this paper is to estimate the corrosion behavior of sintered and heat-treated 17-4 PH stainless steels. In the present study, the influence of aging temperature on corrosion resistance of 17-4 PH stainless steel in dilute sulfuric acid solution has been investigated by utilizing open-circuit potential measurement and potentiodynamic polarization technique.

Experimental Procedure

A water-atomized powder of 17-4 PH martensitic precipitation-hardening stainless steel (corresponding with standards: ASTM-A564 grade 630; UNS S17400) supplied by AMETEK was used. It should be noted that the investigated powder contained addition of lubricant in form of Acrawax in quantity of 0.75 wt.% chemical composition of 17-4 PH stainless steel powder (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 was 2.54 (g/cm2), while its flow was 31 (s/50 g). Typical powder particle size was below 150 µm.

The 17-4 PH steel powder was pressed in rigid die under 600 MPa into cylindrical specimens (20 mm of diameter and 5 mm of height). Then, samples were sintered at 1340 °C in hydrogen atmosphere. The sintering process was carried out in a laboratory Nabertherm P330 furnace. The time for isothermal sintering was 30 min. A heating rate to reach the sintering temperature as well as cooling rate from sintering temperature was 10 °C/min. In order to remove the lubricant, the samples were held at a temperature of 400 °C for 60 min during heating. To compare results of investigation, some as-sintered samples were left (designation A). The remaining sintered specimens were solution treated at 1020 and 1040 °C for 30 min and oil quenched. And then they were aged at three temperatures, namely 480, 490 and 500 °C, for 1 h. Description of the samples designation is shown in Table 1. It refers to preparation conditions of samples used in studies.

Table 1 Description of the samples designation
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The density and porosity of investigated steels were measured by the water displacement method (according to demands of PN-EN ISO 2738:2001 norm). The hardness (HV10) was determined.

Corrosion characteristics are commonly used and considered as one of the important research methods for evaluating corrosion behavior of materials in aggressive environments. In order to estimate the corrosion behavior of both as-sintered and aged 17-4 PH stainless steels, electrochemical study was performed. The potentiodynamic tests were conducted using ATLAS 0531 UI&IA potentiostat, featuring a conventional three-electrode electrochemical cell. This cell consisted of working, reference and counter electrodes. The specimen was the working electrode. The area exposed was 1.33 cm2. Saturated calomel electrode (SCE) was chosen as the reference electrode. Platinum electrode was used as the counter electrode. All experiments were carried out in 0.5 M H2SO4 solution at room temperature. This solution was prepared from analytical grade 97%·H2SO4 and distilled water. All samples were grinded, then degreased with acetone, washed with distilled water and dried prior to tests. Open-circuit potential (OCP) measurement and potentiodynamic polarization test were performed. OCP is often used as a criterion for the corrosion behavior of materials in a corrosive medium. The open-circuit potential (OCP) of investigated steels was investigated during 2 h of immersion in solution. The potential of the samples was recorded as a function of time. After the specimens reached a stable value of open-circuit potential, the potentiodynamic measurements were made. The potential of the electrode was swept at a rate of 1 mV/s from an initial potential of −250 mV versus the OCP to a final potential of 1500 mV. Based on the anodic potentiodynamic polarization curves, corrosion parameters were determined. Corrosion potential (E corr) and corrosion current density (I corr) were evaluated by Tafel extrapolation method. The polarization resistance (R pol) was determined using linear polarization method (called Stern method) and Tafel extrapolate method (called Stern–Geary method). The corrosion rate in (mm/y) was calculated from the corrosion current density (Ref 27).

Metallographic cross sections were prepared using standard procedures. Namely, the samples were cross sectioned, mounted, ground, polished and etched (aqua regia reagent). The microstructural study was done with Nikon Eclipse ME 600P light optical microscope and scanning electron microscopy (SEM). Besides, EDS analysis was carried out in order to determine the chemical composition of studied materials before and after corrosion test. Mainly, SEM and EDS were used to study the surface state of working electrode.

Results and Discussion

The results of density and porosity measurements show that after sintering process 17-4 PH stainless steel has good density and low porosity. The density of sintered 17-4 PH stainless steel was 7.46 g/cm3, which means that the relative density reached quite high value (almost 98%). The open and total porosity was about 0.6 and 2.3%, respectively. As far as the effect on heat treatment on physical properties is concerned, it can be observed that aging temperature does not influence the density and porosity of 17-4 PH steel. Regardless of the conditions of solution treatment followed by aging, the difference in values of both density and porosity is in the second place after the decimal point.

The examples of the microstructure for sintered as well aged 17-4 PH stainless steel are presented in Fig. 1 and 2.

Fig. 1
figure 1

Microstructure of sintered 17-4 PH stainless steel

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Fig. 2
figure 2

Microstructure of heat-treated 17-4 PH stainless steel (solution treatment at 1040 °C, aging at 480 °C

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The heat treatment did not cause a distinct change in the morphology of porosity of 17-4 PH stainless steel. It can be observed that the shape of pores is similar to spheroidal. But after heat treatment, the pores are a little smaller and the amount of them is higher. The microstructure of sintered 17-4 PH stainless steel consists of the martensite matrix and δ-ferrite. The microhardness of martensite in as-sintered 17-4 PH stainless steel is equal to about 294 HV0.01. In the case of as-aged 17-4 PH stainless steel, the microhardness of martensite is higher, and furthermore, the higher aging temperature from 480 to 500 °C and the lower microhardness from about 445 to 395 HV0.01, respectively, are obtained.

The effect of aging temperature on the hardness of studied steel was investigated. Figure 3 shows the hardness of 17-4 PH stainless steel as a function of the heat treatment conditions. It should be pointed that the hardness of as-sintered 17-4 PH stainless steel (specimen A) is equal to 240 HV10. As it might be expected the values of hardness of aged 17-4 PH stainless steels are higher when compared with hardness value of sintered steel. The increase in hardness of the steel can be ascribed by microstructural changes, due to precipitation strengthening effect of fine and dispersed ε-Cu phase in martensite matrix [as in the case of wrought 17-4 PH stainless steel (Ref 5, 12)]. Furthermore, when aging temperature increases hardness of investigated steels decreases for both solution treatment temperatures. Nevertheless the higher temperature of solution treatment, the higher hardness of aged steel was obtained.

Fig. 3
figure 3

Effect of aging temperature on hardness of 17-4 PH stainless steel

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It is well known that the corrosion resistance of steels such as 17-4 PH is associated with their ability to passivation. The passivation means an increase in resistance to corrosion by oxidation and formation of protective film on the surface.

The open-circuit potential as a corrosion parameter indicates the thermodynamical tendency of a material to electrochemical oxidation in a corrosive medium. It is well known that the open-circuit potential may vary with time. This is because of changes in the nature of the surface of the electrode. So oxidation, formation of the passive layer or resistance may occur.

The variations in open-circuit potential for all investigated steels immersed in 0.5 M H2SO4 solution were monitored. Some obtained results are presented in Fig 4 and 5. The values of open-circuit potential at the beginning of the test and after 2 h of immersion in 0.5 M H2SO4 solution were determined for all the samples.

Fig. 4
figure 4

Open-circuit potential variation with time for investigated 17-4 PH stainless steel

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Fig. 5
figure 5

Variation of open-circuit potential with time for the first 30 min for 17-4 PH stainless steel after solution treatment at 1040 °C followed by aging at 480 °C

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In the case of as-sintered 17-4 PH stainless steel, the variation of open-circuit potential with time is different than that for heat-treated 17-4 PH stainless steel. It should be noted that as-sintered 17-4 PH stainless steel has the lowest OCP at the beginning of test and potential shows the tendency to shift toward more negative values. It seems that the intensity of corrosion at as-sintered 17-4 PH stainless steel is rather substantial. It can be confirmed by the fact of constant change (reduction) of potential value during the test. Afterward, the OCP indicates slower changes with time (the reduction in potential decreases). The variation of open-circuit potential with time is similar for steel after all performed heat treatments. At the beginning of test, it can be seen sudden OCP displacement toward positive values (Fig. 5). Then, the potential decreases slowly and after some period of time it stabilizes around a stationary value. The initial increase in OCP suggests that the formation of the oxide film on the sample surface occurs. This kind of behavior improves corrosion resistance. The constant value of potential suggests the thermodynamical stability of the passive layer and resistance to chemical dissolution in solution.

From Fig. 4, some conclusions can be obtained. The higher aging temperature, the higher open-circuit potential. Regardless of conditions of solution treatment, 17-4 PH stainless steel aged at 480 °C reaches the highest values of OCP. As far as corrosion behavior is concerned, sintered 17-4 PH stainless steel behaves worse than the same steel after solution treatment followed by aging. It can be stated that heat treatment improves corrosion resistance of investigated 17-4 PH stainless steel.

Potentiodynamic polarization study in 0.5 M H2SO4 solution was performed in order to obtain an information of the corrosion behavior of the sintered 17-4 PH stainless steel after heat treatment under different conditions. Figure 6 shows the registered potentiodynamic curve for the sintered as well as aged at different temperatures 17-4 PH stainless steel.

Fig. 6
figure 6

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

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As can be seen (Fig. 6), the polarization curve for as-sintered 17-4 PH stainless steel is different from curves registered for the same steel after heat treatment. Although, comparison of the registered curves reveals that cathodic Tafel lines are nearly identical for all investigated materials. However, the same observation cannot be made with respect to the anodic Tafel lines. In the case of as-sintered 17-4 PH stainless steel, potentiodynamic polarization studies demonstrate that: a clearly visible active-passive peak does not appear, a wide passive range occurs and the values of anodic current density are 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 be observed active-passive transition maximum. A region of activation, passivation and transpassivation can be separated in polarization curves. Moreover, two stages of passive process can be observed. Initially, low-potential passive region and then high-potential passive region appear. After this, the increase in current density occurs. It is related to onset of transpassive dissolution.

The parameters such as corrosion potential (E corr), corrosion current density (I corr), polarization resistance (R pol) evaluated based on polarization curves as well as corrosion rate have been calculated for all investigated materials. The R pol was determined using Stern method as well as Stern–Geary method. The electrochemical data are summarized in Table 2.

Table 2 Values of corrosion parameters for the investigated 17-4 PH steel
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As expected, the conditions of sample preparation affected the parameters such as potential corrosion and corrosion current density. The E corr of as-sintered 17-4 PH stainless steel is located at −0.388 (V versus SCE). While, the value of corrosion current density amounts to 2.36E-04 (A/cm2) and is the lowest as compared to the values of I corr assessed for steel after heat treatment. In the case of heat-treated 17-4 PH stainless steel, the corrosion potential is shifted toward to more positive values and the corrosion current density becomes smaller with decreasing aging temperature from 500 up to 480 °C. The lowest value of corrosion current density was obtained for 17-4 PH stainless steel after solution treatment at 1040 °C and aging at 480 °C.

The results in Table 2 indicate that the polarization resistance of 17-4 PH stainless steel (regardless of the used method of determination) changes as a function of the heat treatment conditions. It can be noticed that R pol is reduced with increasing aging temperature from 480 up to 500 °C. This tendency is observed regardless of the temperature of solution treatment. Comparison of the values of polarization resistance revealed that the highest resistance to corrosion was obtained for as-sintered 17-4 PH stainless steel after solution treatment at 1040 °C followed by aging at 480 °C.

Obtained results also demonstrate that solution treatment at 1040 °C followed by aging at 480 °C resulted in the lowest corrosion rate. On the other hand, un-treated 17-4 PH stainless steel reached the highest corrosion rate. It can be observed that the corrosion rate tends to increase when aging temperature increases.

Figures 7 and 8 show SEM images of surface of as-sintered 17-4 PH stainless steel before and after its immersion in 0.5 M H2SO4 solution. EDS analyses were performed in order to distinguish the differences in chemical composition in the specimens. The microanalysis of chemical composition was performed at different regions, and results are shown in Tables 3 and 4.

Fig. 7
figure 7

SEM microstructure of surface of sintered 17-4 PH stainless steel before its immersion in 0.5 M H2SO4 solution

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Fig. 8
figure 8

SEM microstructure of surface of sintered 17-4 PH stainless steel after its immersion in 0.5 M H2SO4 solution

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Table 3 Results of EDS analysis of surface of sintered 17-4 PH stainless steel before its immersion in 0.5 M H2SO4 solution
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Table 4 Results of EDS analysis of surface of sintered 17-4 PH stainless steel after its immersion in 0.5 M H2SO4 solution
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In the case of as-sintered 17-4 PH stainless steel, area numbered 1 was designated in the martensite matrix, while area numbered 2 was δ-ferrite. Microanalysis of chemical composition (Table 3) indicates that the main elements are Fe, Cr, Ni and Cu in both areas. In the case of sintered 17-4 PH stainless steel after its immersion in 0.5 M H2SO4 solution, the analysis was performed at points 1, 2 and 3. Microanalysis of chemical composition (Table 4) indicated that the main elements in point number 3 are Fe, Cr, Ni and Cu. The contents of these elements are almost the same as in the chemical composition of 17-4 PH steel. The main elements in point number 1 and 2 are O, Fe, S and Cr. It should be noted that the contents of elements such as Fe, Cr, Ni and Cu in point 1 as well as 2 are lower than in point 3.

Finally, in the case of sintered stainless steels processed via conventional powder metallurgy routes the corrosion resistance can be improved significantly with increasing density. The undesired effect of pores is assigned to large internal surface areas of sintered parts, which, at the typical structural parts, are still two orders of magnitude larger than their exterior geometric surface areas and therefore can be subject to increased general corrosion. Furthermore, the pores attributed to a lack of passivation within the pores of a sintered part. Therefore, the corrosion in an acid environment can be considered as the action of a hydrogen concentration cell between the external surface of a part and its internal pore surface. The surface of the pores acts as the anode and the engineering surface as the cathode. Metal dissolution occurs primarily in the interior of the material, and after the increasing time, the activation process comes to an end and the potential increases.

Based on the obtained results, the following electrochemical reactions can be proposed in the case of uniform corrosion in sulfuric acid. The electrochemical reaction at the surface includes the anodic dissolution of iron according to the following equations:

$$\left( {\text{oxidation of iron to ferrous ions}} \right)\;{\text{Fe}} \to {\text{Fe}}^{2 + } + 2{\text{e}}^{ - }$$
(1)
$$\left( {\text{oxidation of ferrous ions to form ferric ions}} \right)\;{\text{Fe}}^{2 + } \to {\text{Fe}}^{3 + } + {\text{e}}^{ - }$$
(2)

and the formation of a passive film

$$3{\text{H}}_{2} {\text{O}} + 2{\text{Fe}}^{3 + } \to {\text{Fe}}_{2} {\text{O}}_{3} + 6{\text{H}}^{ + }$$
(3)

While, the cathodic reactions are the hydrogen evolution reaction

$$2\;{\text{H}}^{ + } + 2{\text{e}}^{ - } \to {\text{H}}_{2}$$
(4)

and the direct reduction in sulfuric acid

$${\text{H}}_{2} {\text{SO}}_{4} \to {\text{H}}^{ + } + {\text{HSO}}_{4}^{ - }$$
(5)
$${\text{HSO}}_{4}^{ - } \to {\text{H}}^{ + } + {\text{SO}}_{4}^{2 - }$$
(6)

The aqueous electrolytic solution contains ferrous and ferric ions in sulfuric acid. The dissociation of sulfuric acid causes the formation of the following ions: H+, Fe2+, Fe3+, SO4 2− and HSO4 −.

Formation of a ferrous sulfate salt:

$$4{\text{H}}_{2} {\text{O}} + {\text{Fe}}^{2 + } + {\text{SO}}_{4}^{2 - } \to {\text{FeSO}}_{4} \cdot4{\text{H}}_{2} {\text{O}}$$
(7)

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

Conclusion

The general corrosion behavior of sintered 17-4 PH stainless steel processed under different heat treatment conditions in 0.5 M H2SO4 solution at ambient temperature was studied by open-circuit potential measurement and potentiodynamic polarization technique. In particular, the influence of aging temperature on corrosion resistance of sintered 17-4 PH steel has been studied.

The corrosion resistance was evaluated based on corrosion parameters, such as potential corrosion, corrosion current density polarization resistance as well as corrosion rate. The results showed that the precipitation-hardening treatment can be improved significantly the corrosion resistance of the sintered 17-4 PH steel in diluted sulfuric acid. As far as the influence of aging temperature on corrosion behavior of the sintered 17-4 PH steel is concerned, it can be concluded that the highest corrosion resistance in 0.5 M H2SO4 solution exhibits 17-4 PH after solution treatment at 1040 °C followed by aging at 480 °C. It can be concluded that formation of a ferrous sulfate salt plays a role in the passivation of iron alloys in sulfuric acid.