Passivity Martensitic steels

As dealt with in previous ehapters, the eorrosion resistanee of stainless steel is due to passivation by a surface film of chromium oxide. The chromium content is higher than about 11%, and the low-temperature corrosion resistance as well as the resistance to oxidation and mill scale formation at high temperature increase with increasing content of chromium. Pure chromium steels are either ferritic (low C-content, non-hardenable by heat treatment) or martensitic (traditionally higher C-content for most grades, hardenable by heat treatment). Wifli sufficient content of Ni the structure becomes austenitic, which gives increased formability, weldability. 

For passivation treatments other than for scale removal, less aggressive acid solutions are used. The purpose of these treatments is to remove any contaminants that may be on the component s surface that could prevent the formation of the oxide layer locally. The most common contaminant is embedded or free iron particles from forming or machining tools. A10% nitric add solution is effective in removing free iron. For martensitic, ferretic, and predpitation-hardening grades, a nitric acid solution inhibited with sodium dichromate is used so as not to attack the stainless steel too aggressively. 

The passive CPS consists of absorber elements in 12 fuel assemblies of the first core zone and 36 fuel assemblies of the second core zone. The passive absorber rods are kept in the same position below the core as the active absorber rods using special triggers that are bimetallic plates made of steel of ferritic-martensitic and austenitic grades with differing coefficients of thermal expansion. The trigger is installed on top of the fuel assembly central tube. When coolant temperatures at the fuel assembly outlet exceed 900 K, the thermal deformation of a trigger reaches its critical value leading to release of the shaft and flow up of the passive absorber rod. 

Passive films. Austenitic stainless steels and hardenable stainless steels such as martensitic, precipitation hardening, and maraging stainless steels are seldom coated, but their corrosion resistance depends on the formation of naturally occurring transparent oxide films. These films may be impaired by surface contaminants such as organic compoimds or metallic or inorganic materials. Treatments are available for these materials to clean and degrease surfaces and produce imiform protective oxide films under controlled conditions. These usually involve immersion in an aqueous solution of nitric acid and a dichromate solution. 

---