Strain ageing and corrosion resistance of pipe steel

Abstract

An estimate is given of the effect of strain ageing on the corrosion resistance of pipe steel 17GS in a corrosive operating medium. Metal structure and mechanical properties after strain ageing are considered in two conditions, i.e., after controlled rolling and after normalizing. It is shown that "aged" steel after controlled rolling exhibits greater corrosion resistance than normalized steel.As is well known [1], the majority of pipe steels of grades for oil and gas are subject during operation to strain ageing, which is reflected, on the one hand, by an increase in strength properties, and, on the other hand, by a marked reduction in steel resistance to failure (embrittlement). The main strain ageing mechanisms are known, whereas the connection of this phenomenon with metal corrosion resistance has not been studied sufficiently.The aim of this work is to evaluate the effect of strain ageing on corrosion resistance of pipe steel in an operating medium.Tests were performed on pipe steel 17GS (composition, %: 0.18 C, 0.41 Si, 1.2 Mn, 0.013 P, 0.016 S, 0.025 Cr, 0.080 Ni, 0.013 Cu). The steel was studied in two original conditions: after controlled rolling and after normalizing. Metal ageing was carried out by strain in tension (10%) followed by tempering with T = 250°C for 1 h and air cooling (GOST 7268).Specimens of "aged" steel had a prismatic shape with a size of 12 × 12 × 60 mm with a working face in the form of a microsection (11 × 12 mm) in contact with the corrosive medium.Before the start of tests, the working face was degreased and subjected to acid etching in 5M HCl in order to remove surface damage after machining. The nonworking surface of specimens was coated with chemically resistant insulating material.The corrosive agent used was 30% NaCl solution in distilled water with pH = 4. Experiments were performed in two stages: corrosion tests; coulometric recording of the corrosion products. During corrosion tests in a corrosive medium (volume V = 60 ml; stirring rate v = 20-25 cm/sec), the working face of a specimen was immersed and held successively for 0.5, 1, 1.5, 2, 3, and 4 h. A "one-time" specimen was used for each time point.The corrosion products formed at the working face during tests were removed in a solution of specially remover with pH = 3.8. In parallel, in order to determine background corrosion indices in the removal solution, a control "aged" specimen was exposed to it for 1 h without mixing. The solutions obtained for the test medium and remover with "aged" specimen corrosion products were stored in 0.1M HCl, after which coulometric analysis was carried out.