Petroleum and natural gas produced by oil fields and gas fields contain corrosive gases such as carbon dioxide (CO2, carbonic acid gas) and hydrogen sulfide (H2S) as associated gases, so they are corrosive with respect to metals. Accordingly, excellent corrosion resistance is required of steel materials used in pipelines for transporting fluids, such as petroleum and natural gas, having this high corrosiveness. Typical examples of such corrosion include uniform (general) corrosion, sulfide stress cracking (SSC), and stress corrosion cracking (SCC).
For uniform corrosion, it is known that the addition of Cr to steel is effective at decreasing the corrosion rate. Therefore, in a high temperature environment containing carbon dioxide, the countermeasure of increasing the Cr content of steel has been taken. A specific example of a material having excellent resistance to such corrosion is a martensitic stainless steel such as 13 Cr steel.
However, with martensitic stainless steel, SSC sometimes occurs in an environment containing a minute amount of hydrogen sulfide. For such corrosion, from in the past, it has been known that resistance to SSC in an environment containing hydrogen sulfide can be improved by adding a suitable amount of Mo and Ni to steel to stabilize a corrosion resistant film formed on the surface of the steel. It is also known that the resistance to SSC of welded joints can be improved by using a low C martensitic stainless steel in which the C content of the base metal is decreased with the object of suppressing an increase in hardness in the heat affected zones (HAZ) of welds which decrease resistance to SSC (see Corrosion/96 No. 58).
It has been thought that a low C martensitic stainless steel generally has low susceptibility to SCC. This is because SCC is thought to be sensitized by the formation of a Cr-depleted zone caused by the formation of Cr carbides, and it is more difficult for a Cr-depleted zone to form with a low C martensitic stainless steel than with an austenitic stainless steel. In fact, up to now, it has been thought that SCC does not take place with a low C martensitic stainless steel in a high temperature carbon dioxide environment, i.e., an environment with a high temperature on the order of 80–200° C. and containing chloride ions and carbon dioxide (CO2) and which is also referred to as a sweet environment (referred to below simply as a “high temperature CO2 environment”).
Even with a martensitic stainless steel, as described, for example, in paragraph 0008 of JP-A 07-179943 (1995), if C is added in excess of 0.05%, a Cr-depleted zone is formed resulting from the formation of a large amount of Cr carbides, and the resistance to corrosion by carbon dioxide deteriorates. Therefore, SCC referred to in this specification is of course premised on a C content of at most 0.05%.