What Is Stainless Steel? A Layer Too Thin to See Doing All the Work

Add chromium above roughly ten and a half percent to steel and the surface behaviour changes completely. Chromium's oxide is different from iron's in a decisive way: iron oxide is bulky, porous and flakes off, exposing fresh metal so rusting continues until the object is consumed, whereas chromium oxide is dense, adherent and continuous, and once formed it blocks oxygen and water from reaching the metal. The film is a few nanometres thick, invisible, and forms in moments. Its critical property is self-repair, so scratching through it exposes fresh chromium that oxidises immediately and reseals the gap, which is why a stainless surface can be cut and machined without losing its protection. The corollary is that the film needs oxygen to form and to heal, so stainless steel corrodes where oxygen is excluded, notably in crevices, under gaskets, beneath deposits and inside tight joints, and crevice corrosion is a leading cause of failure in service. Chloride ions, meaning salt, attack the film locally and cause pitting, which is why marine and coastal environments demand particular grades.

The common designations refer to distinct alloy families with different structures and quite different behaviour:

• •Austenitic grades, principally 304 and 316, containing nickel as well as chromium, which are non-magnetic, very formable, weldable and account for most production
• •Grade 316, which adds molybdenum for markedly better resistance to chlorides and is therefore the marine, coastal and food-processing choice
• •Ferritic grades such as 430, containing little or no nickel, which are magnetic, cheaper and less formable, and are used for trim, appliances and exhausts
• •Martensitic grades such as 420, which can be hardened by heat treatment and hold an edge, making them the knife and tool family at the cost of lower corrosion resistance
• •Duplex grades, a mixed structure combining high strength with good chloride resistance, used in chemical plant and structures
• •Precipitation hardening grades, used where very high strength is required alongside corrosion resistance

The name overpromises and the failures are predictable. Free iron contamination is the commonest cause of a rusty stainless surface, since particles from ordinary steel tools, wool or grinding dust embed in the surface and rust there, producing spots that look like the stainless itself has failed when it has not, and the remedy is passivation with an acid that dissolves the iron and restores the film. Chlorides cause pitting, so salt left to dry on a surface is genuinely damaging and coastal installations need higher grades. Crevices trap stagnant liquid where oxygen is depleted. Welding produces heat tint and can leave a chromium-depleted zone next to the weld, which is why weld areas are pickled and passivated and why low carbon variants exist. High temperatures and chlorides together can cause stress corrosion cracking, which is dangerous because it produces cracks with little warning. Cleaning with chlorine bleach on stainless is a common and avoidable mistake.

Several people arrived at chromium steels in the early twentieth century, and the usual attribution to a Sheffield metallurgist in 1913 reflects that he recognised what he had, having been looking for an erosion-resistant alloy for gun barrels and noticing that his rejected samples had not rusted on the bench. German work on chromium-nickel austenitic alloys followed closely, and French and American claims are also defensible, so the honest account is that the material emerged from several laboratories investigating the same alloy system with better analytical tools than their predecessors had. Its adoption was driven first by cutlery, which was the obvious application for a steel that did not stain from food acids, then by chemical plant, food processing, transport and architecture. It is among the most recycled materials in existence, with new production containing a high proportion of scrap, because the alloying elements are valuable enough that recovery is economic without any regulatory push.

Chromium above about ten and a half percent oxidises instantly into a dense film a few nanometres thick that seals the surface and reforms whenever it is scratched. That film needs oxygen, so the metal corrodes in crevices where oxygen is excluded, and chlorides attack it locally and cause pitting. Grade 316 adds molybdenum for salt resistance. Most rust spots on a stainless surface are embedded particles of ordinary steel, not the alloy failing.