ENVIRONMENTAL CORROSION STUDIES OF COLD … III/IJAET VOL III ISSUE I... · ENVIRONMENTAL CORROSION...

International Journal of Advanced Engineering Technology E-ISSN 0976-3945

IJAET/Vol.III/ Issue I/January-March, 2012/75-79

Research Article

ENVIRONMENTAL CORROSION STUDIES OF COLD ROLLED

AUSTENITIC STAINLESS STEEL Manoranjan Kumar Singh and Anil Kumar*

Address for Correspondence

National Institute of Foundry & Forge Technology, Hatia, Ranchi-834 003 India

ABSTRACT Austenitic stainless steels exhibit a unique combination of strength and ductility, which makes them an attractive material for the applications where high strength and good formability along with good corrosion resistance properties are required. The cold rolled stainless steel leads to very good mechanical and chemical properties due to enhanced dislocation densities, increased residual stresses, strain hardening and for the presence of some amount of deformation induced metastable martensite phase. In this paper authors tried to correlate the increase in hardness and corrosion resistance properties in atmospheric and in chloride ion containing corrosive environment of 0 to 80 % cold rolled samples in non sensitized and sensitized conditions. KEY WORDS AISI 304, Cold rolling, deformation induced martensite, atmospheric corrosion, total immersion test

INTRODUCTION

Austenitic stainless steels usually exhibit excellent corrosion resistance, toughness, ductility, low thermal and electrical conductivity and good weldability. However, the strength level, particularly yield strength is relatively low, which is about 200 Mpa, in the annealed state. In order to increase their strength, austenitic stainless steels are often cold worked after solution annealing. Cold working is a convenient strengthening method since austenitic stainless steels normally have a high strain-hardening coefficient. During plastic deformation, depending on the steel composition, the cold working variables, and stacking faults, two different martensitic phase, ε-martensite (HCP) and α’- martensite (BCC) can be formed [J.Talonen.,et al 2005]. The α’ phase is often called strain-induced martensite because it is produced by a diffusionless phase transformation. The most probable way of the phase transformation in the 300 series austenitic stainless steels is the: γ→ε→α’or γ→α’[H. Shin., et al 2001]. These kinds of steels are widely used in chemical, petrochemical, machinery, automobile, nuclear and ship yard industries [A. Lebedev., et al 2000]. The cold rolling effect on stainless steels also leads to increase in dislocation density, vacancy, residual stresses and volume fraction of strain-induced α'-martensite phase in 304L SS depending on the rolling conditions (B Ravi Kumar., et al.2007, Domankova Maria, et al.,2007, Tsubakino H., et al.2004, Raghuvir Singh, et al.2003). In addition to good mechanical properties, corrosion resistant properties are equally importance. The atmospheric environments are classified as rural, urban, industrial, marine, or combinations of these (Naeemi A.H et.al1984, Money K. L, 1987). Atmospheric Corrosion Test is very important test to study the effect of industrial environment on cold rolled stainless steel (G-92-86 Re-approved 1992). The environment has a big effect on the corrosion of stainless steel. The application of stainless steel in the atmosphere is mostly for decorative cladding in architecture. Slight corrosion will cause serious damage on the appearance of buildings. Therefore, the study of corrosion of stainless steels in the atmosphere has been another criterion in recent years (Khanna A.S

et.al 1982) to evaluate corrosion resistance properties. Conventional atmospheric parameters that may lead to metal corrosion comprise of weathering factors such as temperature, moisture, rainfall, solar radiation, wind velocity etc. Air pollutants such as sulphur dioxide, hydrogen sulphide, oxides of nitrogen, chlorides have also been found to contribute to atmospheric corrosion (Brown P.W 1982). This paper investigates the co-relation between cold-working and environmental corrosion resistance of stainless steels (Salvago G, et.al 1983, Stefec R et.al. 1978, Mazza B et.al.1979, Kamachi Mudali U et.al, 2002, Barbucci A et.al, 2002). The aim of these investigations was to determine the influence of the degree of plastic deformation in the cold rolling process on the structure and hardness properties of austenitic stainless steel 304. In particular, special attention was given on the amount of martensite α' phase retaining in the structure of tested stainless steels after cold rolling and also to study the atmospheric and immersion corrosion properties in non sensitized and sensitized samples at 600o C for 1 hour, 5 hours and 30 hours. EXPERIMENTAL

Material processing

Stainless steel AISI 304 having chemical composition of 0.05% C, 18.54% Cr, 9.8% Ni, 1.8% Mn, 0.54% Si by weight was used. The plates were solution annealed at 1100OC for 1 hour to Homogenize and then cut into various plate sizes for cold rolling operation. The original thickness of the material was 10 mm. The stainless steel plates were processed for uniaxial cold rolling from 10-80% reduction in thickness. The samples were cut into blocks measuring 2.0 cm x 2.0 cm thickness from each cold rolled plate. Hardness:

Hardness and micro-hardness measurements of the investigated cold rolled austenitic stainless steel 304 plates were made by Vicker’s method on metallographic samples using the microhardness tester. The microhardness measurements were carried out using a load value about 10 g and time amounting to 10s. The microhardness measurements were realized on metallographic micro-sections in bright area of the austenite grains and grains with small parallel lines characteristic for the martensitic α’ phase.



Atmospheric Corrosion Test Atmospheric Corrosion Test is very important test to study the industrial environment effect on the materials. In this environment different type of industrial gases like sulphur dioxide, carbon dioxide, nitrogen oxides and other gases in the atmosphere vary from place to place depending on the dense of industries, and making the atmosphere very aggressive conditions for corrosion. Stainless steel coupons with a size 2x2 cm taken from cold rolled plates were exposed. The exposed coupons were hung in a hanging corrosion rack, which were facing at an angle 45o to the surface. Three sets of coupons were tested for the evaluation of atmospheric corrosion by weight loss method. The corrosion rates were obtained by using the formula given below (Garcia C et.al 2001). 7290W = Corrosion rate (mm/month)/ D x A x T Where, W=Weight loss (in gm) A= Area of specimen (in cm2) D= Density (in gm/cm3) for AISI 304SS, D=7.9 gm/cm3 T=Time of exposure (in hours)

Total Immersion Test:

For the total immersion test, specimens were grinded up to 1000 grit emery paper and cloth polished from one micron Alumina paste. In total immersion test, samples were fully immersed in 3.5% sodium chloride solution for 2770 hours to asses the no. of pits and the corrosion rate. RESULTS AND DISCUSSION

Deformation Induced Martensite:

The extents of martensitic phase were observed in optical microscope of deformed and un-deformed samples. It is shown in figure 1. In optical microstructure, the etchants were used to bring out the martensite phase as a dark constituent and white constituents as the austenite phase. The small amount of black features seen in Fig.1 (B) is due to the develop of martensite phase in the matrix. The amounts of the dark phases i.e., percentage of martensite phases increases on increasing the deformation of cold rolled samples. These are shown in Fig. 1(C) and Fig. 1(D). The observation reveals that the martensite phase is a function of cold roll processing of the austenitic stainless steel 304.

Fig.1 A 0% cold rolled sample;

Mag. 200x

Fig.1B 20% cold rolled sample;

Mag. 200x

Fig.1C 40% cold rolled sample;

Mag. 200x

Fig.1D 60% cold rolled sample;

Mag. 200x Fig. 1E 80% cold rolled sample;

mag. 1000x

Figure1: Microstructure of cold rolled austenitic stainless steel 304



Fig. 2. The influence of the % cold rolled deformation on the values of HV for the investigated

Steel

y = 2.8537x + 254.62

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80 90

Percentage cold rolling

Hardness, HV

Hardness of material:

The austenite grain size of the investigated as-received corrosion resistant austenitic stainless steel 304 is about 18 µm and microhardness is about 208 HV. In as received condition, the steel structure does not exhibit deformation. The average diameter of the austenite grains in the sample after solution annealing at temperature 1100o C are about 73µm (Fig.1A). After 20 % cold deformation, a structure of austenite grains with the deformation effect of the inside grains were found (Fig.1B) with larger deformation observed along elongated γ grains in the direction of rolling. In the steel AISI 304 after cold rolling with a 20 % and 40 %, a few areas of parallel plates characteristic for martensite α phase were observed (Fig. 1C). These areas are characterized by a hardness of about 360 HV and 390 HV. Metallographic observations of the steel sample deformed with a 60 % to 80% show the large areas of elongated austenite with small parallel lines of martensite α phase (Fig. 1D and 1E). The hardness in these areas amounts to about 415 HV and 468HV. It has been found that during cold rolling process with an increasing degree of deformation; the α phase is formed, which causes an essential size reduction of the steel structure and increase in strain hardening. The occurrence of martensite α phase in austenitic stainless steel confirms the results of mechanical investigations. The measured hardness results is a function of cold working reduction in thickness of the samples as shown in Fig. 2

Atmospheric Corrosion Test:

Stainless steel coupons were exposed in open atmosphere for 17875 hours (more than 2 years). The corrosion rate decreased with increased percentage cold rolled deformation because the martensite and austenite have different corrosion potential, it is easy for them to become the anode and cathode respectively. According to this theory the corrosion resistance of metastable austenite stainless steel will decrease when martensite appears (E.Otero et al 1995, Xu Chunchun et al 2004, A. Kurc et al 2009, Z. Kerner et al 2007). The results were reveled in the Figure 3. It shows that

the corrosion rate of cold deformation of stainless steel significantly decrease with increasing cold deformation. The sensitized samples at 600o C for 1 hour and 5 hours showed no schematic trend of corrosion rate found as shown in Fig. 4 & 5. The overall corrosion rate is decreasing in both cases sensitization as well as non sensitized on increasing cold reduction. The 60% cold rolled sensitized samples showed highly corrosion rate in comparison to deformed non sensitized samples as shown in Fig 4 & 5. The mechanism is not clear.



Fig.3: Atmospheric corrosion of 0-80 % cold rolled for non-sensitized samples

Fig 4: Atmospheric corrosion of 0-80 % cold rolled non-sensitized and sensitized samples at 600

oC/1hrs


oC/5hrs


oC/30hrs



Fig 7: Corrosion rate of 0-80 % cold rolled samples in Immersion test

We have observed that the trend of corrosion rate is decreasing on increasing cold working for sensitized (at 600oC/ 30hrs) and non sensitized samples. For the sensitized sample critical time 30 hours may be responsible the trend corrosion rate starts decreasing after initial increase as shown in Figure 6. The possible reason may be due to the increasing diffusion of Cr to form chromium carbide on increasing cold working. The variation in the corrosion rate with cold rolling was systematic for the received as well as sensitized samples except for 60 % cold rolled samples in 600oC for 1 and 5 hours. The samples were exposed in industrial environment on the roof of the one of the hostel of National Institute of Foundry and Forge technology, Ranchi, India. They suffer due to the presence of atmospheric variables like climate condition, relative humidity, surface condition and polluted environment. Total Immersion Test

Samples were immersed in 3.5 %sodium chloride solution for 2770 hours at room temperature. It is found that the corrosion rate decreased with increasing cold rolling deformation as shown in Fig 7.This is due to increases hardness with increases cold rolled deformation. CONCLUSIONS

• Deformation induced martensite increases in cold rolling with 0 to 80 % reduction of thickness of the investigated samples and hardness increased from 208HV to 468HV.

• Atmospheric corrosion rate decreases with increasing reduction in thickness of the investigated samples by cold rolled deformation.

• The atmospheric corrosion rate of sensitized samples at 600oC for 1hr & 5hrs decreasing on increasing cold working & while that sensitized for 30 hrs also decreases but initial increase in comparision to nonsensitized.

• Total immersion test shows corrosion rate decreases with increasing deformation of cold rolled investigated samples.

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2. H. Shin, T. Ha, Y. Chang, Kinetics of deformation induced martensitic transformation in a 304 stainless steel, Scripta Materialia 45 (2001) 823-829.

3. A. Lebedev, V. Kosarchuk, Influence of phases transformations on the mechanical properties of austenitic stainless steels, International Journal of Plasticity 16 (2000) 749- 767.

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