"ferritic and martensitic stainless steel"
Question:
"I'm in the city bus business near Montreal, Canada. The new buses that we bought have a ferritic and martensitic stainless steel structure - a combination of 41003, 30403 and 20400. The structure is essentially made of tubular HSS. After 3 months of use, the stainless steel structure began to rust(during winter,here there is a lot of calcium on the road). Should I be concerned about that? Usually a bus should last 16 years. Do we have to do something to protect the structure as we do with a carbon steel structure? Or will the rust stay at a superficial level? What details do you need to be able to tell if the 41003 stainless steel will continue to corrode or not? Which conditions will affect the corrosion of the 41003?"
Answer:
You have referred to three different stainless steels comprising the bus structure, all with different corrosion resistances. Of the three, 41003 should be the least resistant to corrosion pitting. Without more details, it is not possible to tell for certain whether the steels will continue to corrode or not. However, given the right conditions, the 41003 may experience continued weight loss with time. Information needed includes detailed knowledge of the environment, i.e., duration and extent of exposure, wet/dry cycles, temperature, exposure to salts, type of salts etc.(more than can typically be done justice in a discussion forum format). The anticipated most aggressive corrosive species are chloride salts. It should be understood that the presence of water is required.