Assessed Laboratory Work Pitting of stainless steelAnswers:1. Pitting resistance equivalent is a measurement of steel resistance to pitting and crevice corrosion. Chromium, molybdenum and nitrogen concentration in steel are influencing the value of PRE. For example, austenitic stainless steel with equal or less than 3% Mo. Formula: PRE = %Cr + 3.3 x %Mo + 30 x %N. If PRE value is equal or more than 40, its exhibit good resistance to pitting. 2. Composition of 304-type steels is Fe -18-20%Cr-8-12%Ni-0.1%N and 316-type of steels is Fe-16-18%Cr-10-14%Ni-2-3%Mo-0.1%N. 316-type of stainless steels has higher resistance to pitting, according to PRE value formula. Additional of Mo in 316-type stainless steel increased PRE value due to Mo element reduced active dissolution of stainless steels. 3. Electrolyte conditions within the pit will gradually acidify and hydrolysis. There are excess of positive charges in pit area causing migration of aggressive anions into the pit and high concentration of H+ ions due to hydrolysis process compared to bulk electrolyte condition. Oxygen in bulk electrolyte will reduced to OH- ions which increase pH of electrolyte. 4. Pitting potential is influenced by the concentration of anions in electrolyte. Na2SO4 is an inhibitor solution with non-aggressive SO42- anions. Higher concentration of SO42- anions increased pitting potential due to lower acidity than Cl- anions. Meanwhile, Na2S2O3 lowered pitting potential because of unstable S2O32- anions undergo reduction reaction to S2- and SO32-. These ions cooperate with S2O32- anions to breakdown the metal passivity layer and promote pitting corrosion. 5. Rough surface Smooth surface

Stainless steels with rough surface finish are prone to more pitting corrosion and lowered pitting potential. A rougher surface has more surface area that could sustain the conditions for active dissolution of metals compared to smooth surface. More defects exit in rougher surface as a form of preexisting pit in which propagation of pit is easier and number of sites for pit initiation also increased. 6. without Crevice with Crevice

Immersion of stainless steel in 0.1M NaCl resulting to pitting corrosion. In the case of small gap existing between steel surface and fastening washer in the same solution causing crevice corrosion. From the graph observation, crevice initiation is dependent on the corrosion potential of stainless steel surface. In the absence of crevice, critical potential of pitting is higher than for crevice corrosion. In pitting corrosion mode, it is generally started with initiation time followed by propagation. Meanwhile, the initiation time for crevice corrosion is either much shorter or nonexistent because of the mechanism of crevice corrosion. 7. Pitting potential at scan rates 20mV/s is lower than polarization under 1mV/s. Potential scan rate would influence the initial pit geometry and pit environment resulting change in potential pitting. At lower scan rates, pit environment swift toward less aggressive conditions. Meanwhile, by using a high scan rate reduced or does not allow time for local environment to modify. 8. Mechanism of pitting corrosion started with dissolution of passive film and gradual acidification of the electrolyte due to aggressive anion penetrate into film. Present of Cl- anions will accelerate depassivation mechanisms by O2- diffuse outward from the film. In an aerated chloride environment, stainless steel has sufficient of oxygen anions to maintain its passive layer or repassivate the pitting area. Presence of peroxide will increase the pH of solution cause by peroxide reduction reaction. Overall reactions of hydrogen peroxide: H2O2 + 2H+ + 2e- -> 2H2OOpen circuit potential of stainless steels shifted in the noble direction but decreased pitting potential. Metal is less susceptible to pitting under this circumstances.