The influence of microstructure on the corrosion behaviour of ferritic-martensitic steel

Abstract

Modern steel alloys are composed of several phases, for example ferrite and martensite. By varying the fraction or the size of the grains of these phases, the mechanical properties of the alloy can be altered. These microstructural variations were already found before to influence the corrosion properties of the alloy; this has been investigated in this master thesis by means of electrochemical experiments. The prior austenite grain size has been varied for fully martensitic samples; it was found that grain boundaries are less noble sites in the lattice and that an increase in prior austenite grain boundary density would increase the corrosion rate. Grain boundaries were also found to be anodic initiation sites for pits. However, since the fine microstructures have more initiation sites, fewer pits would grow critical due to the lack of compensating cathode area. The depth and size of the pits in the fine-grained material were found to be larger than those in the coarse-grained samples. When the amount of ferrite in the ferritic-martensitic samples was increased, the corrosion potential became more negative. The ferrite formed a galvanic couple with the martensite and corroded preferentially due to its lower (more negative) corrosion potential. The corrosion current density was found to peak at a certain ferrite fraction. However, the anodic dissolution rate of the ferrite was found to decrease with increasing ferrite fractions, since the cathode-to-anode area ratio became less favourable for galvanic corrosion. The nucleation of ferrite also partitioned more alloying elements to the austenite, during annealing, due to the higher solubility of those elements in austenite. The alloying elements were found to sacrificially corrode for the iron, forming a protective layer of reaction products on the martensite surface. The corrosion potential for the martensite was found to increase with increasing amounts of alloying elements, while the current density decreased and the corrosion rate was retarded. These changing electrochemical properties of the martensite also influenced the corrosion properties of the ferritic-martensitic alloy on the macroscopic scale.