Evaluation of galvanic corrosion in mild steel, 309S and 409M alloy combinations for marine environment applications

Abstract The objective of this study is to simulate galvanic corrosion in 409M, 309S -& MS couples in light of field applications and to compare with the corrosion rates of the individual grades. Metals undergo corrosion when exposed to outdoor environments. But the rate of corrosion gets accelerated when the metals are connected to form a galvanic couple and are exposed to marine environments. One such galvanic system being used in aggressive environments like marine applications can be combinations of Mild steel (MS)- 409M (Ferritic Stainless Steel) & 309S (Austenitic Stainless Steel). The electrochemical tests which include measurement of voltage/current w.r.t time, potentiodynamic polarization studies and electrochemical impedance measurements using 5% NaCl solution were designed in order to assess how these alloys behave (i) individually and (ii) in galvanically-coupled states. Visual observations, determination of corrosion rates, microstructural inspections and Scanning Electro Microscopy (SEM) analysis were carried out as a part of corrosion evaluation method. The results indicated that the galvanic coupling of dissimilar metals significantly accelerates the corrosion rate of the less noble metal, particularly in the MS-309S couple where the difference in their corrosion potentials between the two alloys is as high as 450 mV. The corrosion rate of MS coupled with 309S increased to 0.277 mm yr −1 , which is approximately 6.9 times more compared to the MS-MS couple. The comparison was done with respect to MS-MS system. The test data shall be used to compare the relative corrosion resistances of (i) MS - MS (ii) MS - 409M (iii) 409M—409M (iv) 309S−409M (v) 309S–MS and (vi) 309S—309S. This is required to understand the relative corrosion resistances of alloys during field application and to arrive at a suitable solution for maximizing its service life.