INTERGRANULAR STRESS CORROSION CRACKING SENSITIZED AISI 304 STAINLESS STEEL IN A MgCl2 SOLUTION AND ITS INHIBITION POSSIBILITY

• Main Author: DILASARI (NIM : 22110034), BONITA
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/19965
• Institution: Institut Teknologi Bandung

Maintaining austenitic stainless steel at temperature ranged from 425 to 8150C for long period can result in the formation of chromium rich carbide especially at grain boundary. As a consequence, chromium depletion zone area is formed adjacent to the carbide and the formation of suitable protective film cannot occur. In case of the carbon concentration in the alloy is high (&#65374;0,08% or higher), the formation of chromium rich carbide will occur along the grain boundaries and the alloy becomes susceptible to intergranular corrosion. The attack of this corrosion will initiate IGSCC on the stainless steel that withstands static tensile load. With respect to the results of previous SCC investigations with static tensile load method, the relationship between elongation and time during the test can be divided by three regions. In the secondary region, the elongation rate will be constant with time and it is known as a steady state elongation rate and has applied as a parameter for the resistance of material to SCC. <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> A series of experiments has been conducted to study the effect of sensitization on the intergranular corrosion susceptibility of AISI 304 SS in 242,08 gpl MgCl2, and the possibility to inhibit the localized corrosion which initiate IGSCC by NaNO2 and CaSO4 addition. Resistance to localized corrosion of the alloy in the various test solutions might be evaluated with respect to its passive film breakdown potentials. Sensitization of specimens has been conducted at 6500C for 24 hours in an argon atmosphere inside a tube furnace. Measurement of cyclic potentiodynamics was done using scan rate of 0.167 mV/second in a test solution acidified with HCl to pH 3 at room temperature, 75 and 850C. The measurement results has indicated that an increase of NaNO2 and CaSO4 concentration can extend the passive film breakdown potential and reduce passive current density which lead to reducing intergranular corrosion susceptibility of the stainless steel. The relationship between the concentration ratio of aggresive and inhibitif ions in logarithmic scale and the passive film breakdown potential can be expressed by equation Enp = a + b log( Ca/Cin). The a and b constants obtained are -32,816 mV and -54,291 mV vs Ag/AgCl for NaNO2 inhibitor and -289,27 mV and 14,644 mV vs Ag/AgCl for CaSO4 inhibitor respectively. <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> Constant tensile load SCC tests have been conducted on sensitized AISI 304 SS in aerated 242,08 gpl MgCl2 solutions at 75 and 850C. Sensitization has resulted in intergranular brittle fracture on AISI 304 SS containing 0.05% C or higher, immersed in an inhibitor free test solution. The concentration of carbon in stainless steel (exhibited by more intensive ditch structure along grain boundaries) affects the intergranular corrosion susceptibility of the alloy. Therefore an AISI 304 SS containing higher carbon concentration will be more susceptible to IGSCC. CaSO4, Na2SO3 and NaNO2 inhibitors are then added into the MgCl2 solution to produce test solutions containing different inhibitor concentrations which are further used to study the influence of inhibitor to the SCC resistance of sensitized AISI 304 SS. The extend of the elongation of each specimen under constant tensile load in test solution, is recorded as function of time. The steady state elongation rate, tss and tf parameters obtained from each SCC test are then used as parameters to compare IGSCC susceptibility of the stainless steel in each test solution containing different inhibitors. It is revealed that the steady state elongation rate of alloy is proportional to CaSO4 concentration and hence increasing CaSO4 inhibitor concentration will result in shorter time of fracture. Obviously, the addition of 0.605 gpl CaSO4.2H2O or higher will increase the SCC susceptibility of immersed alloy. In contrast, NaNO2 addition is able to decrease the steady state elongation rate of alloy and can avoid alloy fracture at least during SCC test. <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> Based on polarization measurement and SCC test results as well as fracture morphology examination which is carried out under SEM, it is concluded that IGSCC is initiated by passive film breakdown on the area adjacent to grain boundaries and this results in intergranular corrosion of alloy if repassivation cannot occur. Stress concentration just in front of the crevice tip causes crack propagation to a certain depth as soon as local stress intensity has &#8805; than its critical value. Test solution will then penetrate into the crack which then causes anodic dissolution to occur at the crack tip. Because of the crack propagation rate is faster than the crack tip dissolution rate, a sharp crack will be formed and this will limit the rate of dissolution at the crack tip. The dissolution rate is being controlled by the rate of outward mass transfer of cations which are accumulated inside the crack adjacent the crack tip. Presence of active slip due to tensile stress concentration at the crack tip will result in the formation of plastic strain at the crack tip. Similarly with the previous step, if the local stress intensity has been &#8805; its critical value, crack will continue to propagate up to a certain depth. Crack propagation along the grain boundary followed by anodic dissolution at the crack tip which are occur repeatedly, has been known as IGSCC and this causes brittle fracture of the alloy.