How does stainless steel work?

Stainless Steel is an Iron alloy which contains a minimum of 10.5% Chromium. The Chromium reacts with Oxygen when present on its surface to form a passive Chromium Oxide film. This prevents the Iron in the stainless from oxidizing and rusting. See how stainless steel works here.

Which grade of stainless is best?

Different grades of stainless steel and which grade to use for an application will largely come down to:

• Corrosive environments - is it in salt water or fresh water? Is it inland or coastal?
• Regularity of washing or rain washing - does it get regular washing (maintenance or rain washing), or is it under cover (no rain washing) and in a salty atmosphere?
• Strength required – what are the stainless steel tensile and yield load strength requirements?
• Temperature of the environment – is the environment ambient or is it a high temperature furnace application?
• Surface finish – the smoother the finish the more corrosion resistant. A large improvement in corrosion resistance performance happens at a surface roughness of about 0.5 micrometer Ra.
• Chemical passivation and or pickling after welding or production - pickling and/or passivation of ground surfaces improves the corrosion resistance by two or more grades.

Here is a guide on stainless steel use and selection.

Which grade is better – 304 or 316 stainless?

316 grade stainless has the element of Molybdenum added (only 2%). This adds a significant improvement to its ability to withstand corrosion over 304 and avoid rusting and mild rust stains often called ‘tea-staining’.

For internal use and environments with a low degree of corrosivity, then 304 grade might be fine. However for external and coastal applications 316 grade might be best. For salt water applications Duplex 2205 grade is suitable if tidal, but Super Duplex 2507 would be required if submerged.

How corrosion resistant is each grade of stainless – what is PREN?

Pitting Corrosion resistance can be estimated and calculated by a PRE value or PREN (Pitting Resistance Equivalent Number), which is an approximate guide for the corrosion resistance of each grade of stainless steel.

Is 316 suitable for seawater/salt water?

What is marine grade stainless? For salt water applications such as being submerged or as keel bolts then Super Duplex 2507 would be more suitable to eliminate the risk of unwanted corrosion. With the increased levels of Chromium and Molybdenum, Super Duplex 2507 is far more immune to pitting corrosion from Chloride attack.

Super Duplex has a PREN value of 40, making it a ‘Super’ grade and ideal for submerged salt-water applications.

For more information or technical advice please contact the Nickel Institute or the Australian Stainless Steel Development Association (ASSDA).