THE EFFECT OF SENSITIZATION TEMPERATURE ON STRESS CORROSION CRACKING OF STAINLESS STEEL AISI 304 IN SULPHATE ACID ENVIRONMENT
I.
Sukmana
& Y. Burhanuddin
Dosen Jurusan Teknik Mesin, Fakultas Teknik-Universitas Lampung Jl.
Prof.
Soemantri Brojonegoro
1,
Bandar Lampung Email:
isukmana
@yahoo.com
ABSTRACT Austenitic stainless steel 304 (AISI 304) has a good corrosion resistant, but a heat treatment process can cause a sensitization phenomenon and the corrosion resistant will decrease. One of the most serious corrosion problems that can occur is
stress corrosion cracking (SCC). This study
is to determine
the
effect
of
sensitization temperatures (i.e. 500 oC, 670 oC, and 840 oC) on the SCC phenomenon of AISI 304 in sulphate acid environment with total immersion method. It was observed for the specimen with sensitization temperatures of 500 oC and 670 oC, the dominant corrosion form is pitting corrosion, where the highest corrosion rate was occur on the specimen with sensitization temperature of 500 oC after 336 hours of exposure. Average of Corrosion Rate of specimen with sensitization temperatures of 500 oC, 670 oC, and 840 oC after 480 hours of exposure are 2.76 mm/y, 1.58 mm/y, and 0.9 mm/y. Stress corrosion cracking occurs on the specimen with sensitization temperature of 840 oC after 480 hours of exposure. Keywords: stress corrosion cracking, sensitization temperature, and total immersion. ABSTRAK Baja tahan karat austenitik AISI 304 memiliki ketahanan korosi yang baik, namun bila mengalami perlakuan panas dapat terjadi fenomena sensitisasi sehingga akan menurunkan ketahanan korosinya. Fenomena korosi yang dapat terjadi terutama adalah korosi retak tegang (SCC). Penelitian ini dilakukan untuk mengetahui efek temperatur sensitisasi (500 oC, 670 oC, dan 840 oC) terhadap fenomena SCC pada AISI 304 di lingkungan asam sulfat dengan metoda perendaman total. Dalam penelitian ini ditemukan bahwa temperatur sensitisasi 500 oC dan 670 oC akan menyebabkan korosi Pitting, angka laju korosi tertinggi ditemukan pada specimen dengan temperatur sensitisasi 500 oC setelah 336 jam perendam. Laju korosi rata-rata setelah 480 jam perendaman untuk temperatur sensitisasi 500 oC, 670 oC, dan 840 oC masing-masing adalah 2.76 mm/y, 1.58 mm/y, dan 0.9 mm/y. Fenomena korosi retak tegang ditemukan pada spesimen dengan temperatur sensitisasi pada 840 oC setelah 480 jam perendaman. Kata kunci: korosi retak tegang, temperatur sensitisasi, dan perendaman total. INTRODUCTION Austenitic Stainless Steels are the most widely applicated steel in industry, about 70-80% of stainlless steel production. AISI 304 is also the most popular Chrom stainless steel that has a good corrosion resistance and mechanical properties. AISI 304 is the iron-based alloy that contains more than 12% Chromium that can
forms a protective oxide film
on
the surface
to hence
the
corrosion resistance.
Oxide film
can regenerate naturally when the surface of steel was damage. However, the precipitation process of Chrom-Carbide due to the heat treatment processes or welding processes can cause the sensitization phenomenon and later will promote a spesific corrosion phenomenon, namely
environmentally induced cracking
(EIC).
Environmentally Induced Cracking (EIC) is general term for brittle mechanical
failure
that
results
from a synergism
of
tensile stress,
susceptible material,
and corrosive environment. Corrosion
rate is
usually quite low and design stresses to cause EIC are often below the yield stress. Stress
Corrosion Cracking is the popular phenomenon at EIC (Jones, 1991). Sensitization is the phenomenon based on the Chromium-depleted zone at the grain boundary regions. For example, in welding processes of various stainless steels, Heat Affected Zone (HAZ) will be sensitized in which cause Chrom (Cr) move to the grain boundaries while at the other regions will have less Chrom even can be considerably below 10 – 12 % that required to create an oxide film. When Cr moves to the grain boundaries, it will precipitates to forms Cr23C6 and the regions with less Cr content will have less corrosion resistant and known as Chromium-depleted zone (Decker & Langer, 1987).
It is known that the
effect
of
sensitization phenomenon of
austenitic stainless steel is
a great importance, especially for the application at Oil Company, chemical and nuclear industry and also shipping company. Previous study has been investigated at boiling water condition and with the H2SO4 environment that the intergranular cracking may arise after 72 hours period of exposure (Jones, 1991). In Austenitic stainless steel, an intermediate temperature range of 425 to 840 0C (840 – 1550 0F) Chrom
carbides, (Fe, Cr)23C6 are insoluble and precipitate at grain boundaries. Above
840 0C
the Chromium carbides are soluble; below
4250C
the diffusion rate of carbon is too low to permit formation of the carbides.
Figure 1 show that
the Chromium carbide precipitates are very high in Chromium, but the matrix alloy is depleted of Chromium in the grain boundaries.
Previous researcher reported the microstructure
of sensitized
austenitic
stainless steel. The
specimen
was
prepared with a solution treatment at 1050 0C
for 1 hour followed by water
quenching. For the sensitization process, samples were immersed in a batch of solution-treated at 670 0C, in a muffle furnace for 4 hours. For the testing of sensitization, the specimen was subjected to the oxalic acid etch test. A ditch microstructure was taken to be confirmation of the sensitization (Singh, 1987). Carbide precipitate (i.e. M23C6) also can induce by the welding process of austenitic stainless stell. It causes Chromium depleted zone near the grain boundaries and then decrease the stress corrosion cracking resistant of the welded stell as reported by Gooch, (1984). Other researcher founded that SCC can occur when the steels are expossed to the “high pH” as well as “near-neutral pH” or “low pH” solution (Fang, et al., 2003).
Stress corrosion cracking of AISI 321 stainless steel
on
acidic chloride
environment also has been reported by other researcher (Huang, 2001). Although massive progresses have been made, the corrosion forms and processes that associated with SCC of austenitic stainless steel on the sulphate acid environment are still not clear. The objective of this study was to determine
the effect of sensitization
temperatures
on
the SCC process
of
austenitic
stainless steel AISI 304
on the
sulphate acid
(H2SO4)
environment.
Three different sensitization temperatures (i.e., 500 0C, 670 0C, and 840 0C) have been chosen in this study. The variations of sensitizations temperature are based on the diagram of Time- temperture-precipitation for M23C6 of AISI 304 as shown in Figure 2 (Sourmail, et al., 2003). Figure 1 Chromium Depleted Zone at the Grain Boundaries (Too, C.H., 2002) (a) Chromium depleted zone and M23C6 precipitates (b) Chrom concentration Figure 2 Time-Temperature-Precipitation for C23C6 in 304 austenitic stainless steel METHODOLOGY Material
and Experimental Procedures Material
that
used in this study was
AISI 304 and specimen was prepared based on ASTM standard (Decker & Langer, 1987) and for preparation, cleaning, and evaluation was based on ASTM G 1 – 72 standards (Annual Book of ASTM, 1979). Generally, a testing specimen was prepared by cutting the AISI 304 industrial standard plate into strips with a specific dimension (Figure 3). For the sensitization process, the strips were then placed on the salt bath furnace that set in three different temperatures (i.e. 500 0C, 670 0C, and 840 0C) for 4 hours of sensitization time and then quenched in water. 3 Bending area 7 38 90 100 Figure 3 Dimension of the specimen To clean the oxide film at the surface that caused by sensitisation process, specimen was immersed on the aquadestilata solution containing 10% (volume/volume) of HNO3 for 1 hour at temperature of 60 0C (Annual Book of ASTM, 1979). U-Bend Corrosion Test Specimen After the cleaning process, the strips were then
bent approximately
1800
around a predetermined radius and maintained in this plastically
(or
elastically) deformed condition during the
SCC
test.
The bending method was based on ASTM G30 and the final specimen is shown in Figure 4. Figure 4 Final U Bend Specimen Exposure Method There are two types of SCC test expossure as suggested by the ASTM Standard, i.e. cyclic immersion and total immersion (Annual Book of ASTM, 1979). This study was conducted with total immersion method, where the specimen was immersed on the suphate acid until the evaluation time as shown in Figure 5 (not the exact scale). Holder H2SO4 inside the glass test Specimen Bench Work Figure 5 Total Immersion Method Weight Loss Evaluation The calculation of
the average corrosion rate can
obtain with
the
Weight Loss method based on
the
ASTM
standard as follows (Jones, 1991):
Corrosion Rate = (K × W)/(A × T × D)..
1
Where: K = a constant. T = time of exposure in hours to the nearest 0.01 h A = area in cm2 to the nearest
of
0.01 cm2 W = mass loss in g, to nearest 1 mg D = density in g/cm3.
Corrosion Forms and Micro Structure To better understanding of the initiation and propagation process of EIC, the evaluation will be based on the corrosion forms and micro-struture. Generally, several corrosion forms will associated with EIC of austenitic stainless steel at acidic environments, there are: (a) Uniform corrosion, (b) Pitting corrosion, and (c) Intergranular, and (d) SCC. The evaluation of uniform and pitting corrosion will be based on the macro structure evaluation while inter granular and stress corrosion cracking were by the micro-structure analysis. Pitting corrosion will be determined by the number of cases per surface area and the average deep of pitting’s penetrations. RESULT AND DISCUSSION Comparison between the structure of non- sensitized and sensitized steels (i.e. 500 0C, 670 0C, and 840 0C) was presented in Figure 6. The micro structure of non-sensitized steel indicates the regular forms at the grain boundaries region, while on the sensitized ones indicate the dich structure. Higher sensitization temperature tends to produce dicher grain boundaries. It can be hypothesized that the black dots on the grain boundaries and ditch structures were associated with the precipitation of Cr23C6 as also suggested by other researchers (Huang, 2001; Sourmil, 2003; and Too, 2002).
(a) (b) (c) (d) Figure
6 Micro Stucture
of
Stainless Steel
AISI 304
(magnification 360x) (a) Non-Sensitized (b) Sensitized at 500 0C (c) 670 0C, and (d) 840 0C Table 1 Result of the Corrosion Test Sensitization Time of Expossure Weight Loss Corrosion Average of Temperature (0C) (Hours) (grams) Rate (mm/y) CR (mm/y) 48 0.19 2.1 500 192 0.37 1.02 336 2.93 4.49 480 3.23 3.44 48 0.19 2.1 670 192 0.33 0.9 336 0.88 1.35 480 1.83 1.95 48 0.14 1.51 840 192 0.15 0.41 336 0.19 0.29 480 1.29 1.4 2.76 1.58 0.90 Data of weight loss, Corrosion Rate (CR) as well as average of CR over the exposure time were presented on Table 1. The CR was calculated based on equation (1). Weight loss trend during the exposure time for 3 different sensitization temperatures can be presented as relation between weight loss and time of exposure as shown in Figure 7. 3,5
Weight Loss (gram) 3 2,5 2 1,5 1
0,5 0 0 200 400 600 Time of Exposure (Hours) Sensitized Temperature at 500 0C Sensitized Temperature at 670 0C Sensitized Temperature at 840 0C Figure 7 Trend of Weight Loss during Exposure Table 2 Corrosion Forms and the Regions Sensitized Time of Expossure Corrosion Temperature (Hours) Corrosion Forms Regions 48 Pitting Peak dan 15o 500 oC 192 Pitting 15o, 30o 336 Pitting Peak, 15o, 30o 480 Pitting Peak, 15o, 30o 48 Pitting 15o 670 oC 192 Pitting 15o, 30o 336 Pitting Peak, 15o, 30o 480 Pitting 15o, 30o 48 Uniform All surface 840 oC 192 Pitting 15o, 30o 336 Pitting Peak, 15o, 30o Pitting, Inter- 480 granular & SCC Peak, 15o, 30o Based on Table 1 above, the highest CR is 4.49 mm/y where it was occur on the specimen with sensitization temperature of 500 0C after 336 hours of exposure. The average CR number of sensitization temperature of 500 0C (CR = 2.76) was higher than 600 0C (CR = 1.58) and 800 0C (CR = 0.90) respectively. Based on Figure 7 above, until 192 hours of exposure, the trend of weight loss of every sensitized specimen were relatively equal. After 192 hours, the specimen that sensitized on 500 0C gives the highest weight loss number. Since
the weight loss
number
was calculated
based on
the
data
of
uniform corrosion,
the
other corrosion forms will be evaluated separately Table 3 Penetration of Pitting Corrosion after 480 hours of exposure Time of Exposure Sensitized Number of Average (Hours) Temperature (oC) Pitting Corr. Penetration 500 68 0.07% 480 670 147 0.14% 840 138 0.12% F igure 8 SCC and IG Corrosion of sensitized specimen at 840 0C (magnification 180x) Data of the forms and corrosion region that evaluated with the macro and micro-structure analyses was presented on Table 2. The Evaluation of penetration of pitting corrosion after 480 hours of exposure is shown on Table 3. Based on Table 2 above, stress corrosion cracking and IG corrosion were not occur on the specimen that sensitized at 500 0C or 670 0C but only pitting corrosion, even after 480 hours of exposure. For specimen that sensitized at 840 0C, SCC was initiated by uniform after 48 hours of exposure, followed by pitting and finally inter- granular and SCC corrosion after 480 hours. The inter-granular as well as SCC forms that founded in this study are presented in Figure 8. It can be hypothesed that the sensitization temperature of 840 0C is the most potential one on inducing the EIC
of stainless steel AISI 304
when exposed on
sulphate acid environment.
Based on Table 3 above, the sensitization temperature of 670 0C was give the highest number of pitting corrosion as well as average pitting penetration, followed by 840 0C and 500 0C respectively SCC process and the corrosion forms during the exposure time of sensitized steel at themperature of 840 0C, can be described as follow: Uniform Corrosion ? Pitting nucleation ? pitting growth ? Inter-Granular ? SCC. Our finding of the continous process of stress corrosion cracking that initiated by several corrosion forms was in good agreements with results that had already reported by other researchers (Meng,. et al., 2003 and Krawiec,. et al., 2006). CONCLUSIONS This study reported the validation and process of SCC of austenictic stainless steel AISI 304 with various sensitized temperatures and expossed at sulphate acid environment and it can be concluded as follows: 1. Sensitized temperature of 500 0C give the higest average of corrosion rate (2.76 mm/y) and followed by 670 0C (1.58 mm/y) and 840 0C (0.9 mm/y). 2. Dominant corrosion form of specimen sensitized at 500 0C and 670 0C is pitting REFERENCES Annual Book of ASTM, 1979, Preparing & Evaluating Corrosion Test Specimens, G1–72 pp. 781 – 868. Decker, R.F. & Langer, E.L., 1987, ASM Metals Handbook of Corrosion, Vol. 13, 9th Edition, ASM International, Ohio. Fang, B.Y. et. al., 2003, Review of Stress Corrosion Cracking of Pipeline Steel in “low pH” and “high pH” solutions, Journal of Material Science, Vol. 38, pp. 127-132. Huang, Y., 2001, Stress Corrosion Cracking of AISI 321 Stainless Stell in Acidic Chloride Solution, Journal of Material Science, Vol. 25, pp. 47-51. Jones, D.A., 1991, Principles and Prevention of Corrosion, 2nd Edition, MacMillan Publishing, New York. Krawiek, H. et al., 2006, Dissolution of Chromium- Enriched Inclusions and Pitting Corrosion of Resulfurized Stainless Steel, Metallurgical and Materials Transcastions, Vol. 37A, pp. 1541-1549. corrosion without inter-granular as well as SCC, even after 480 hours of exposure. Inter-Granular and SCC forms were occur on the specimen that sensitized at temperature of 840 0C. Meng, Q. et al., 2003, Stainless-steel Corrosion and MnS inclusions, Nature, Vol. 424, pp. 389- 390. Singh, P.M & Malhotra, S.N., 1987, Stress Corrosion Cracking Susceptibility of various Austenitic Stainless Steel in Polythionic Acid, NACE Corrosion Journal, Vol 43 No. 1 pp.26 – 31. Sourmail, T. et al., 2003, Sensitisation and Evolution of Chromium-depleted Zones in Fe-Cr-Ni-C Systems, ISIJ International, Vol. 43, pp. 1814-1820. Too, C.H., 2002, Sensitistion of Austenitic Stainless Steel, Master Thesis, University of Chambridge, Chambridge. (Artikel diterima tgl 15/02/07, disetujui tgl 1/4/07, direvisi tgl 7/6/2007) JURNAL TEKNIK/Volume 14 Nomor. 2 AGUSTUS 2007 ISSN 0854 - 2139 115 116 JURNAL TEKNIK/Volume 14 Nomor. 2 AGUSTUS 2007 ISSN 0854 - 2139 JURNAL TEKNIK/Volume 14 Nomor. 2 AGUSTUS 2007 ISSN 0854 - 2139 117 118 JURNAL TEKNIK/Volume 14 Nomor. 2 AGUSTUS 2007 ISSN 0854 - 2139 JURNAL TEKNIK/Volume 14 Nomor. 2 AGUSTUS 2007 ISSN 0854 - 2139 119 120 JURNAL TEKNIK/Volume 14 Nomor. 2 AGUSTUS 2007 ISSN 0854 - 2139 JURNAL TEKNIK/Volume 14 Nomor. 2 AGUSTUS 2007 ISSN 0854 - 2139 121 122 JURNAL TEKNIK/Volume 14 Nomor. 2 AGUSTUS 2007 ISSN 0854 - 2139
