Localised corrosion of high chromium superferritic alloys

Abstract

The general and pitting corrosion resistance of a selection of alloys based on a composition of Fe–40Cr were studied. These alloys have been investigated for corrosion resistant applications, specifically by microalloying with platinum group metals (PGM) to induce cathodic modification. The corrosion environments considered were a 10% H₂SO₄solution, and a synthetic mine water containing 500 ppm Cl⁻ + 300 ppm SO₄ ²⁻. The effects of three metallurgical variables on the localised corrosion processes were investigated, these being: prestraining by cold work to increase the strain energy; recovery annealing to give a dislocation substructure; and alloying with 0·2 wt-% ruthenium. Prestraining by cold deformation of ∼5% does not impair the passivity or pitting resistance. However, a recovered dislocation substructure promotes localised attack which destabilises the passivity in a reducing acid medium. The addition of the cathodic element ruthenium increases the resistance to localised attack, but promotes restoration by recovery at the normal recrystallisation temperature, forcing a loss of corrosion resistance. The alloys all exhibit excellent pitting resistance in the chloride environment.