API 5l steel pipe is most commonly made by using a duplex stainless steel or a grade of iron that has higher nickel content, such as a six percent of Mo material or high nickel alloys. Ferrite stainless steels are very resistant to API steel pipe but grades may vary with equivalent pitting resistances. What is more, the austenitic grades of API 5l steel pipe have other major drawbacks since corrosion is preferential at the grain boundaries. It is also commonly associated with welding since stainless steel can be heated into a sensitizing temperature range. In fact, some high end piping products can be formed in the heat affected zone of a weld since chromium can combine with carbon in the steel. Thus, chromium carbides will be formed in a solid manner in the metal grain boundaries. Around these particles there is an area in which the material is placed in a corrosive environment.
The most common way today to avoid damage to API 5l steel pipe is to specify a low carbon grade of high end piping products. This is especially true with stainless steel when welding is going on. In the past, when it is difficult for mills to achieve low carbon levels, titanium or niobium can be added into high end piping products.
This is because these elements preferentially combine with carbon in API steel pipe. Grades of API 5l steel pipe contain these additions even though galvanic corrosion can occur when different metals are in contact. This is especially true for high end piping products that are placed in an electrically conductive liquid. Stainless steel is not normally corroded in the galvanic couple since API steel pipe is usually more corrosion resistant.
