The Effect of Mill Scale on OCTG Sour Cracking Resistance

Abstract There exists a commonly-held belief that as-delivered oil country tubular goods (OCTG) possess superior resistance to sulfide stress cracking (SSC) than standardized testing for SSC may identify. This belief relies on the nature of the mill scale present on OCTG being a barrier to corrosion and hydrogen permeation that would act to retard SSC. Despite the popularity of this belief, limited data assessing this hypothesis exists in the public domain. A laboratory study was conducted to assess both the corrosion behavior and hydrogen permeation through API(1) C110 material in two conditions: 1) mill scale removed with typical NACE(2) TM0177 surface finish; and 2) mill scale intact. The study was performed on samples with and without applied stress in NACE TM0177 Solution B with 7% H2S/N2 gas purge. The findings indicated that samples with mill scale intact exhibited both higher corrosion rates and higher hydrogen flux in the conditions assessed. Based on the results of the study, it was hypothesized that the higher surface roughness on as-received material contributes to the higher apparent corrosion rate and hydrogen permeation. Per the outcome of this work, reliance on mill scale for either corrosion or cracking protection is considered ill-advised.