Grade 304 and Marine Grade 316 stainless steel both contain similar amounts of chromium which give them their anti-corrosive properties, but Grade 304 is more susceptible to corrosion than Marine Grade 316 because it lacks one important extra ingredient.
Grade 304 Stainless Steel
Grade 304 is very versatile and is widely used in a variety of outdoor applications including electrical enclosures, auto moulding and trim, wheel covers, kitchen appliances, hose clamps, exhaust manifolds, storage tanks, and piping.
Its combination of 18% chromium and 8% nickel also give it good resistance to moderately acidic or caustic solutions, making it suitable for non-severe applications such as kitchen and food environments. It’s a product that is easy to fabricate, easy to clean and comes in a variety of finishes and appearances.
However, Grade 304 is still susceptible to corrosion from chloride solutions found in saline environments. Here chloride ions can create localised areas of corrosion known as ‘pitting’, which can spread beneath the stainless steel’s chromium barrier and cause further corrosion. Evidence of such corrosion can be seen in a discolouration that appears on its surface, known as ‘tea staining’.
Marine Grade 316 Stainless Steel
Marine grade 316 stainless steel is similar to Grade 304 with one notable exception. As well as chromium and nickel, Marine grade 316 also contains 2% to 3% of molybdenum, a hard, silver white metallic element used to toughen steels and increase corrosion-resistance in nickel alloys.
The addition of this ingredient gives Marine grade 316 superior corrosion resistance to other chromium-nickel steels when exposed to chemical corrosives such as seawater and brine solutions. Like Grade 304, it is also durable, easy to fabricate, clean, weld and finish, but unlike Grade 304, it provides greater protection from salt spray and brine.