Inhibition of pitting corrosion of AISI 316L dyeing vessels by SO4--and NO3-ions

Reactive dyes are the most frequently used chemicals in modern fiber dyeing processes. In order to drive the dyes onto fibers a proper amount of NaCl is added to dyeing bath. Despite AISI grade SS316L has a superb resistance to general corrosion in most environments it can be prone to pitting and cevice corrosion in the presence of Cl- ions with detrimental affects on passive films. The most commonly used practice for avoiding a localized corrosion such as pitting is to make use of inhibitors. By adding inhibitors into corrosive solutions in contact with metals and alloys to be protected, generally a fixation of damaged passive surface, raising of the corrosion potential, and making new passive films over surfaces is achieved. General inhibitor types used for corrosion prevention involves organic molecules including amine groups, hydrogen evolution poisons, scavangers, vapor phase inhibitors and some salts of oxyanions like SO4-2, NO3-, CrO4-2. In the present work a couple inorganic inhibitors have been tested for suppressing/avoiding pitting corrosion and corrosion fatigue of fiber dyeing tanks made of AISI grade stainless steel 316L. Since the damaging affect of Cl- ions on passive films of stainless steels is a known phenomenon, a partial replacement of Cl- ions in original dyeing solution with the proposed inhibitors have been carried out, and prepared solutions have been tested with potentiodynamic-galvanostatic cyclic polarization method. Na2SO4 and NaNO3 anionic salts have been chosen to be tested for their inhibition efficiency on the steel. The reason for preferring the proposed inhibitors, in addition to their pitting preventiveness and ineffectiveness on dyeing quality, lies in their negatively charged anions which can function in the same way of Cl- ions as they drive dye molecules to fibers. Open circuit potential and potentiodynamic-galvanostatic cyclic polarization tests of AISI 316L have been carried out in Na2SO4 - NaCl, NaNO3 ? NaCl and Na2SO4 ? NaNO3 compound mixes of various concentrations and deaerated medium at 70 °C. Potentiodynamic-galvanostatic cyclic polarization tests carried out with some of the test solutions with sufficient amount of inhibitors have shown prevention affect on pitting corrosion to a degree. A complementary SEM surface analysis has revealed correlations between test solutions with inhibitors, and type of pitts and corresponding polarization curves. By UV-Visible range spectrophometric analysis of the test solutions an insignificant affect of inhibitors over dyeing quality has been determined. Experimental results and findings have been reported in table and graphics. The results obtained by several experimental method have been evaluated in a comparative manner. Then, a dyeing solution type and composition out of the test solutions, having the best pitting prevention ability and less negative affect on dyeing quality, have been determined. Keywords: Synthetic fiber, inhibitor, 316L stainless steel, potentiodynamic-galvanostatic cyclic polarization test.