What is stainless steel?

Stainless steel is an iron–carbon alloy that contains a minimum of about 10.5% chromium. The chromium reacts with oxygen to form a thin, invisible chromium-oxide layer on the surface. If that layer is scratched or damaged, it re-forms on its own as long as oxygen is present, which is what gives stainless steel its resistance to corrosion, and why the same alloy can look “stainless” for decades in one environment and pit badly in another where that layer can’t rebuild fast enough.

Grades and why so many exist

There are more than 150 grades of stainless steel, of which around fifteen are in common industrial use. They fall into a few families that behave quite differently in service:

• Austenitic grades (304, 316 and their low-carbon “L” variants) are the workhorse of process equipment: non-magnetic, weldable, and good all-round corrosion resistance. 316/316L adds molybdenum, which meaningfully improves resistance to chlorides and is why it’s the default choice near coastal air, in CIP cleaning with chloride-based sanitisers, or in brine and marine-adjacent duty.
• Duplex grades combine an austenitic-ferritic structure for higher strength and better resistance to chloride stress-corrosion cracking than standard austenitics, at higher cost: used where 316 isn’t quite enough.
• Ferritic and martensitic grades are less common in vessels but show up in fasteners, pump shafts and wear components where hardness matters more than fabricability.

The choice of grade for a vessel or reactor depends on the product it will hold, the cleaning regime (chloride-based sanitisers are particularly aggressive), the operating temperature, and whether chlorides are present in the process stream or even just in the local atmosphere.

Surface finish is not cosmetic

For process equipment, finish is a functional specification, not an aesthetic one. A 2B mill finish is adequate for many non-hygienic industrial duties. A brushed (No. 4) or mechanically polished finish reduces surface roughness enough to matter for cleaning validation in food and pharmaceutical service. Electropolishing goes further, removing a thin surface layer to produce a smoother, more chromium-rich surface that resists product adhesion and bacterial attachment: standard practice for pharmaceutical-grade vessels and piping. The finish also has to survive welding: heat-affected zones darken and lose some corrosion resistance until they’re pickled and passivated, which is why that step isn’t optional on a vessel that’s going to hold anything more demanding than water.

Beyond process equipment

Stainless steel is used far beyond process equipment (in aerospace, cutlery, medical and surgical instruments, automotive parts, appliances and construction) and it is fully recyclable: roughly 60% of the stainless steel produced today comes from recycled scrap and end-of-life products, which is one reason its long-run cost of ownership tends to compare well against cheaper materials that need earlier replacement.

For Makfen’s customers, the practical questions are always the same: which grade, which surface finish, and how the welds will be treated. Those three decisions determine how the equipment behaves in service for the next twenty years.