Over the years, the gap between stainless steels and Ni-base alloys has become increasingly diffuse, not only in terms of chemical composition but also when considering performance in corrosive service conditions. While Ni-base alloys can sometimes be considered the safe choice, alternatives with a lower nickel content are often more cost efficient. Due to advances in material technology and the always-increasing knowledge about material performance, high-alloyed stainless steels can indeed be attractive and competitive materials of choice for highly corrosive service conditions.
This work focuses on characterizing various high-alloyed stainless steels and Ni-base alloys in terms of localized corrosion performance, as well as uniform corrosion performance in chloride contaminated sulfuric acid. Corrosion data will be presented to demonstrate differences and similarities between these material groups, and between individual grades. This knowledge can be used to make better-informed decisions when selecting materials for highly corrosive applications.
Stainless steels and Ni-base alloys are often considered as construction materials in applications where highly corrosive conditions are expected. High levels of halides, low pH and high temperatures are factors that often contribute to the selection of such materials.
Ten different alloys have been included in this work, representing a range of highly alloyed stainless steels and Ni-base alloys. The purpose has been to evaluate the corrosion resistance of stainless steels with alloying content in the 6Mo range or higher, and competing Ni-base materials. The austenitic grade N08904 and two super-duplex grades have also been included for reference.
The main characteristics of the selected materials are described in the following section. The description of each material group is in no way exhaustive or definite – it is meant as a brief overview of some commonly used materials, with the focus on the grades that are included in this work.
This material group is often a go-to choice for applications where the corrosion resistance of standard austenitic grades is not sufficient. 6Mo is not a single grade, nor is it a clearly defined group of grades, but it commonly refers to highly alloyed austenitic stainless steels with a molybdenum content of roughly 6 weight%.1 Other common characteristics are chromium and nickel contents of around 20% and a nitrogen content of approximately 0.2%. The two grades 254 SMO† (UNS S31254) and 6XN† (UNS N08367 / N08926) are similar in terms of composition, with the most significant difference being the higher nickel content of the latter.
