Corrosion Cracking of Kh70 Pipe Steel for Potentials Close to the Maximum Protective Potential

Abstract

The susceptibility of Kh70 pipe steel to stress corrosion cracking (SCC) is studied by the method of deformation with slow strain rate. The susceptibility of the analyzed steel to SCC is estimated by using the coefficient KS equal to the ratio of values of the relative narrowing of the specimen in air and in a solution. It is discovered that, for the corrosion potential, the specimens made of Kh70 steel fail faster in the model soil electrolyte than in air. The values of the coefficient KS lie within the interval from 1.02 to 1.24, which indicates that Kh70 steel is not susceptible to SCC under those conditions. As we move from the corrosion potential to – 1.0 V, the regularities of the corrosion-mechanical behavior undergo changes: the specimens suffer tensile fracture almost under the maximum stress, the zones of ductile fracture become much smaller than for the corrosion potential, and the fracture mechanism changes from ductile into brittle with ductile final fracture. The values of the coefficient KS vary within the range 1.65–2.15. Hence, under the indicated conditions, the Kh70 steel is susceptible to SCC. It is shown that the mechanical stresses, corrosive media, and protective potentials are the main factors of SCC, while the other factors are auxiliary (they increase or decrease the influence of the main factors without changing the mechanism of SCC).