Pitting Corrosion Resistance Determination of Duplex Stainless Steel by Using Critical Pitting Temperature Method

Fuad M. Khoshnaw Loughborough University Mechanical and Manufact. Eng. UK

Ramadhan H. Gardi Salahaddin University Hawler Mechanical Engineering Iraq

Abstract

Although duplex stainless steels exhibit excellent corrosion resistance in many severe corrosive environments, their corrosion resistance greatly deteriorated after heat treatment. The influence of heat treatment on pitting corrosion of two stainless steels, SAF 2304 and AISI 316L, was studied. The stainless steels were given aging treatment at temperatures of 400°C and 500°C for various times. The influence of this treatment on the pitting susceptibility of the stainless steel was evaluated by the FeCl3 test in accordance with ASTM standard G48.
The results showed that by increasing the aging temperature from 400°C to 500°C the pitting corrosion rate of stainless steel alloys were increased at all aging times. For stainless steel alloys these results can be attributed to metallurgical aspects such as carbide precipitation, formation of other phases such as (s), secondary austenite, etc., which have significant effects on corrosion behavior in stainless steel alloys. Increasing the amount of these aspects encourages pitting corrosion.

Introduction

Duplex stainless steels are iron-chromium-nickel (Fe-Cr-Ni) alloys, usually with 50% austenitic and 50% ferritic microstructure at room temperature. These steels possess a combination of properties: high strength and corrosion resistance that is not readily attainable using conventional single phase austenitic or ferritic stainless steel [1,2]. Compared to austenitic stainless steel, they have better local and stress corrosion resistance, particularly in hot corrosion environments containing chloride ions. Compared to ferritic stainless steels they can offer improved formability, weldability and toughness. Because of these properties duplex stainless steels are used increasingly in marine applications, some petrochemical industries and oil refineries. Although they have existed since 1930s, they did not become popular before the development of high-alloy duplex stainless steels, because of difficulties in hot workability and their susceptibility to intergranular corrosion after welding and heat treatment [1-3]. During heat treatment of stainless steels for 400°C and 1000°C, various transformations occur involving precipitation of (s), (R) intermetallic phases and (a) body-centered cubic. These transformations seriously affect the localized corrosion resistance such as pitting and intergranular corrosion [4]. The object of this report is to study the effect of heat treatment on pitting corrosion of SAF 2304 and AISI 316L.

Experimental Procedure

The chemical composition of stainless steels SAF 2304 and AISI 316L are given in Table (1). Fig. (1) shows the microstructure for both studied types at a received condition.

Chemical composition%	Cr	Ni	Mo	Mn	C	Si	P	S	N	Ti	Ca	Cu	Fe
SAF 2304 measured	>20.96	5.3	0.139	0.86	0.05	0.893	0.045	0.015	0.11	-	-	0.3028	Ball.
SAF 2304 standard	23	4	-	-	0.03	-	-	-	0.05-0.20	-	-	-	Ball.
AISI 316L measured	16	13	2.7	1.45	0.03	0.48	-	-	-	0.03	0.04	0.49	Ball.
AISI 316L standard	16-18	10-14	2-3	2.0	0.03	1.0	0.045	0.03	0.10	-	-	-	Ball.

To investigate the effect of metallurgical aspects on pitting corrosion of stainless steel alloys, two heat treatments were carried out in this study, 400°C and 500°C. For each of these temperatures five different times were selected: 0.5hr, 1.5hr, 10hr, 24hr and 72hr.

(a)	(b)
Fig. (1) Microstructure of Stainless steel alloys a. SAF 2304(x 1500) b. AISI 316L (x 1500)

Test specimens measuring 20mm x 25mm were cut from a sheet 50x25cm with 4mm thickness for SAF 2304 and 2mm thickness for AISI 316L, with refinishing of the sheared edges. The 25mm dimension had been parallel to the longitudinal direction. Before and after heat treatments the specimens were mechanically finished with the aid of 120 and 220 grid abrasive paper, subsequently using water to avoid overheating. The specimens were degreased using soap and acetone, then dried with hot air.
Each specimen was placed inside a 0.5-litre Erlenmeyer flask after 250ml of 10% FeCl3 solution was poured into the container housed in a constant-temperature water bath. After exposure for 24 hours at appropriate temperature the specimens were removed, rinsed with water and scrubbed with a nylon bristle brush under running water, dried with hot air and examined with the naked eye for pitting.
If pitting did not appear, the same actions were repeated with a temperature of 2.5oC higher than the previous, using fresh solution, and so on until pitting appeared [5].
To investigate metallurgical aspects such as carbide precipitation, brittle phases etc., by microstructure detentions for each heat treatment proceeded in this study, beside any specimens for a given condition, another specimen, measuring 10 x 10 mm, was inserted into the furnace with them to proceed and take the same variations. These investigations were carried out by optical microscope. After each heat treatment the specimens were mounted to give more facilities of preparation stages (grinding, polishing and etching). Polishing has been done using diamond slurry on a medium-nap cloth. The specimens etched chemically according to ASTM standard [6] as shown in Table (2).

Table (2): Etching reagent for stainless steel alloys.

Etching reagent	Composition		Use
1. Ferric chloride and nitric acid	Saturated solution of FeCl3 in HCl, to which a little HNO3 is added etching time equals 2min.		For AISI 316L structure and chromium carbide along the grain boundary
2. Ferricyanide solution	Potassium ferricyanide KOH H2O	30g 30g 60ml	For etching duplex stainless steels to distinguish between ferrite and sigma phase: sigma phase, light blue and ferrite, yellow. Darken carbide containing chromium
	The solution used hot and etching time equals 15 second.
	The specimens immersed for 60sec. in solution A at 70oC, quenched in cold water. Then immersed in solution B for 30sec. then washed in stream of warm water

Results and discussion

Table (3) illustrates the results of critical pitting temperature tests for both SAF 2304 and AISI 316L. As received condition, the SAF 2304 specimen was subjected to pitting at 40°C, but AISI 316L was subjected to pitting at 32.5°C. The higher critical pitting temperature of SAF 2304 is attributed to higher chromium and nitrogen content, or in another words, increasing the chromium and nitrogen contents adhesives of the chromium oxide passive film that formed on the stainless steel alloys. These results agreed with H. Tsug et al., [7], who found that the addition of nitrogen in 22% Cr stainless steel increases the pitting resistance in FeCl3 solution.

First treatment at 400°C

Table (3) and Fig. (2) shows the results of the critical pitting temperature of SAF 2304 and AISI 316L aged at 400°C. Fig. (2) shows that the duplex stainless steel SAF 2304 specimens aged at 400°C for 0.5 hour and 1.5 hours have an equal critical pitting temperature (40°C) as in a resaved condition, i.e. the aging temperature at 400°C for aging times 0.5 hour and 1.5 hours have no effects on pitting resistance of SAF 2304.

Table (3): Critical pitting temperature for SAF 2304 and 304L

Heat Treatment oC	Time	SAF 2304 oC	304L oC
400	0.5 hr.	40	30
	1.5 hr	40	30
	10 hrs	37.5	30
	24 hrs	37.5	27.5
	72 hrs	37.5	27.5
500	0.5 hr.	37.5	27.5
	1.5 hr	37.5	27.5
	10 hrs	37.5	27.5
	24 hrs	37.5	25
	72 hrs	32.5	25

Increasing the aging time to 10, 24 or 72 hours, the critical pitting temperature lowered by 2.5°C and equals 37.5°C, i.e. the corrosion resistance to pitting decreased as the aging time increased. This may be attributed to the greater precipitation of chromium carbide along the ferrite austenite boundaries which resulted at the expense of Cr depletion for the surface.
E.Alfonsson and R. Qvarfort [8] states that the presence of precipitation along ferrite–austenite boundaries in 453E grade duplex stainless steel (26% Cr, 5.5% Ni, and 1.5% Mo) has a bad influence on pitting resistance.
Fig. (2) showed that when the AISI 316L specimen aged at 400°C for 0.5 hour, 1.5 hours and 10 hours, the critical pitting temperature was reduced by 2.5oC compared with as received condition and further reducing in critical pitting temperature observed for specimens aged for 24 hours and 72 hours, which reaches 27.5oC in both cases.
Fig. (3) shows that the pitting that formed on both types of stainless steels after the test. The figure illustrates that the depth of pits in AISI 316L is more than that of SAF 2304. Also, more pits were observed on the AISI 316L specimen aged for 72 hours. Note that the aging times for specimens in Fig. (3) are 0.5, 1.5, 10, 24 and 72 hours from left to right in each row. The same approach repeated for figures 6, 10, 12 and 15.

Fig. (2) Effect of aging times on critical pitting temperature of
SAF 2304 and AISI 316L heat treated at 400°C

In general the temperature 400°C has little influence on the pitting characteristics of duplex stainless steel type SAF 2304, while this temperature has more effects on AISI 316L. This may be attributed to the fact that AISI 316L is easily sensitized but SAF 2304 requires higher temperature to be sensitized.

Fig. (3) Pitting corrosion of heat treated SAF 2304 and AISI 316L at 400°C.
Upper = AISI 316L, Lower = SAF 2304.

Second treatment at 500 oC

Table (3) and Fig. (4) showed that the critical pitting temperature of SAF 2304 and AISI 316L aged at 500°C. This result showed that duplex stainless steel SAF 2304 specimens aged for 0.5 hour, 1.5 hours, 10 hours and 24 hours subjected to pits at a temperature lower than that as received condition by 2.5oC, i.e. 37.5oC, but marked decrease in critical pitting temperature produced by increasing aging time to 72 hours which reaches 32.5oC. This may be attributed to the decomposition of the ferrite phase to secondary phases which revealed darkness, as shown in Fig. (5). The decomposition of ferrite to secondary phases means more Cr dissolution in these new phases, and this resulted in depletion of Cr, particularly at the surface of alloys, thus lowering the critical pitting temperature.
Fig. (4) shows that the AISI 316L specimen exhibits continuous decrease in critical pitting temperature if the aging times are increased to 500°C. This can be explained by the fact that the AISI 316L type was subjected to more sensitization when aging temperature increased from 400°C to 500°C. More effects observed for the specimens aged for 24 and 72 hours, which are subjected to pits at 25oC.

Fig. (4) Effect of aging times on critical pitting temperature of
SAF 2304 and AISI 316L heat treated at 500°C


Fig. (5) Microstructure of SAF 2304 aged at 500°C for 72hours (x1500)

Fig. (6) shows the pit formation on both type of stainless steels after testing. This figure illustrates that the density of pits of AISI 316L specimens aged at 500°C is smaller than the specimens aged at 400°C for the same conditions. This means that the preferential sites for pit initiations are increased due to increasing the precipitation along the grain boundaries, as shown in Fig. (7).

Fig. (6) Pitting corrosion of heat treated SAF 2304 and AISI 316L at 500°C.
Upper = AISI 316L, Lower = SAF 2304

Fig. (7) Microstructure of heat treated AISI 3161L at 500°C for 72 hour (x1500)

Conclusions

1. If the aging temperature was increased from 400°C to 500°C, the susceptibility of SAF 2304 and AISI 316L to pitting corrosion increased.
2. Critical pitting temperature could be sufficient to determine the pitting corrosion resistance in duplex stainless steel.

References

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