Crevice corrosion inhibition of 316L by SO4¯ ¯ and NO3¯ in Cl¯ containing environments

AISI 316L stainless steel commonly used in nuclear, petrochemical, gas and dyeing industries is resistant to general corrosion in many different environments due to the chromium oxide (Cr2O3) passive film on its surface. However, 316L stainless steel is susceptible to crevicing, a localized corrosion type. Spreading of crevice corrosion on metal surfacevaries depending on the crevice forming material, its surface roughness and the torque holding it on the sample. Since metals dissolution proceeds in the areas unseen and undetectable even a very low load or a shear force can cause devastating consequences; a huge loss of time, materials and labor. Measures for corrosion protection can be taken in both ways, reducing the corrosiveness of the environment and increasing the corrosion resistance of the metal. Inhibitor use in small amounts can prevent general and localized corrosion without altering solution functions so much. Inhibitors prevent corrosion by four different mechanisms. These can be applied to the surface of the metal as much as to have a thickness of a souple moleculs the surface precipitate forming surface coatings, metals react to form is caused by corrosion, creating a passive film on the surface of the resulting corrosion products, corrosive ions effect removal (away) from eliminating or corrosive ions environment. Inhibitors are classified according to the mechanism of the anodic compounds and inhibitors, cathodic inhibitors, adsorption inhibitors, corrosion consumers, mixed inhibitors, vapor phase inhibitör and fat souble inhibitors. Inhibitors prevent corrosion by four different mechanisms. They are, in an order, adsorption to metal surfaces to form film of a couple molecule thick, reaction with surface to form a precipitate coat, reaction with the surface to form a passive film, and elimination of dissolving ions by some chemical reactions. Depending on inhibition mechanisms and chemical content inhibitors can be divided into groups such as cathodic, anodic, adsorbtion type, mixed type, scavenger, vapor phase, and oil-soluble inhibitors In this study, crevice corrosion susceptibility of AISI 316L in aqueous solutions of two different inorganic salts was tested by designed crevice formers held on the samples under a predetermined torque value. Since the detrimental effect of chlorine on the passive films is known, chlorine was reduced gradually as replaced with inhibitor. Na2SO4 ve NaNO3 inorganic salts were used as the inhibitors. Open circuit potential (OCP), lineer polarization (LP) and potentiodynamic cyclic polarization (CP) tests of 316L stainless steel were carried out in deaerated mixed compounds such as Na2SO4 – NaCl and NaNO3 – NaCl of various concentrations held at 75 °C, according to the ASTM G5 standard test procedures. The potentiodynamic–galvanostatic cyclic polarization tests done in some test solutions revealed a good degree of inhibition on the crevice corrosion of AISI 316L with both of the inhibitors of sufficient amounts. Experimental results and findings were reported in tables and graphics. The results of all tests were evaluated together. Then, the contents and concentrations of the most efficient mixed compounds on the crevice corrosion inhibition of the steel was determined. Key words: inhibitor, 316L stainless steel, potentiodynamic cyclic polarization (CP).