Influence of Some Metallurgical Factors on the Corrosion Behaviour of Two C–Mn Steels in Acidic Solutions

Abstract

The corrosion behaviour of two hot-rolled C–Mn steels (API 5LX X70 and API 5LX X52) has been studied and compared in de-aerated 0·1 N sulphuric (lcid and in de-aerated 0·1 N hydrochloric acid. The influence of certain metallurgical factors (chemical composition, textures of three planes relative to the rolling direction, ratio of ferrite–pearlite area, ferritic grain size, shape and composition of non-metallic inclusions, extent of banded structures and work-hardening) on the morphology of the attack has been examined both during. and after galvanokinetic polarisation, followed by galvanostatic polarisation (corresponding to a total of 22·5 × 10⁴ C.m⁻²).

The galvanokinetic polarisation curves of each of the two steels were virtually unaffected by the plastic deformation produced by the Charpy impact test. The pearlite distribution, the Cu content and the shape and composition of non-metallic inclusions have an important effect on the anodic dissolution process that showed a strong tendency to start at the Mn sulphide inclusions in the case of the API 5LX X52 steel.

Etch pits were observed on the API 5LX X70 surfaces, particularly on the plane perpendicular to the rolling surface and direction, and on all three planes after the stress-relief thermal treatment at 600°c.