Why stainless steel rusts, and how to prevent it

Stainless steel rusts when its protective chromium oxide layer is beaten by chlorides, contamination or trapped moisture, not because the steel is faulty. The metal defends itself with a thin, self-healing passive film, and rust appears only where that film cannot form or is broken down: in salt-laden air, where carbon steel particles are pressed into the surface, in oxygen-starved crevices, or beside a weld that has been sensitised. Each cause is preventable with the right grade, finish and handling. This guide explains why stainless rusts and how to stop it.

Stainless steel does not rust like carbon steel

Stainless steel contains at least 10.5 percent chromium, and that chromium reacts with oxygen to form an invisible, tightly bonded chromium oxide film across the surface. This passive layer is self-healing: scratch it and, given oxygen, it reforms within moments. It is what stops the underlying iron from rusting the way bare carbon steel does. So stainless is not rust-proof in the absolute sense, it is corrosion-resistant, and its resistance depends entirely on keeping that passive film intact and able to repair itself.

So why does stainless steel still rust?

Rust or staining on stainless always traces back to the passive layer being overwhelmed or prevented from healing. There are four practical causes: chloride pitting, free-iron contamination, crevice corrosion in trapped moisture, and sensitisation beside welds. None is a flaw in the steel itself, and each has a clear defence, so stainless corrosion is almost always a grade, finish or handling problem rather than bad material.

Chloride pitting

Chlorides are the main enemy of stainless steel. Chloride ions, from coastal air, swimming pools, de-icing salt or process chemicals, locally break through the passive film and start small, deep pits. This is why standard 304 stainless can pit and stain in a marine or poolside setting where it looks perfectly clean inland. The defence is molybdenum: 316 stainless contains around 2 to 3 percent molybdenum, which strongly stabilises the passive film against chlorides, and duplex grades resist them further still. Matching the grade to the chloride exposure is the single most important decision. Our guide to 304 and 316 stainless steel sets out the difference.

Free-iron contamination and tea staining

A very common cause of rust spots is contamination, not the stainless itself. When stainless is cut, ground or handled with tools that have touched carbon steel, tiny particles of ordinary iron are pressed into the surface. Those particles rust, leaving brown spots and streaks often called tea staining, and the rust can look as though the stainless has failed when only the surface iron has corroded. The defence is strict separation: dedicated stainless tools, brushes and grinding discs, no carbon steel contact, and passivation to remove any embedded iron. Clean, uncontaminated stainless keeps its finish.

Crevice corrosion in trapped moisture

The passive film needs oxygen to heal, so wherever moisture is trapped and starved of air, stainless becomes vulnerable. Under gaskets, in unsealed laps, beneath deposits and in tight joints, a crevice holds stagnant liquid that cannot re-oxygenate, and corrosion starts inside it, especially if chlorides are also present. The defence is detailing: design out crevices, seal or drain joints, avoid deposit traps and keep surfaces clean. Good design removes most crevice corrosion before it can begin, which is the same principle that protects aluminium, as our guide to whether aluminium corrodes explains.

Sensitisation and weld decay

Heating stainless into the range of roughly 450 to 850 degrees Celsius, as happens beside a weld, can cause chromium carbides to form at the grain boundaries. These carbides lock up chromium and leave a narrow band next to them depleted below the level needed for passivity, so that band corrodes preferentially. This is sensitisation, and the corrosion that follows is called weld decay. The defence is low-carbon steel: L grades such as 304L and 316L keep carbon below 0.03 percent so the carbides cannot form, and stabilised grades such as 316Ti do the same by tying up the carbon. Our guide to welding stainless steel covers this in detail.

How to prevent stainless steel from rusting

Prevention comes down to grade, finish and handling, in that order.

A smoother surface finish matters more than it looks: a polished 2B or brushed surface traps less dirt and moisture than a rough one, so it stays passive and clean. Regular washing in exposed settings removes chloride and dirt build-up before it can attack.

Restoring stainless that has started to rust

Surface rust on stainless can usually be reversed. Light tea staining is removed by cleaning with a proprietary stainless cleaner or a mild acid, never with a carbon steel wire brush, which would add fresh contamination. Heavier contamination and heat tint are removed by pickling, using a nitric and hydrofluoric acid paste that strips the damaged layer, followed by passivation in a nitric or citric acid solution that rebuilds the passive film. Once the surface is clean and passivated, the chromium oxide layer reforms and the steel is protected again.

Choosing the right grade

Most rust problems are really grade-selection problems. 304 suits dry, indoor and general use. 316, with its molybdenum, is the workhorse for coastal, marine, poolside and chemical exposure. Duplex 2205 adds both strength and still greater chloride resistance for demanding structural and marine work. Naming the environment on the specification, coastal, industrial, chemical or inland, lets the right grade be chosen from the start. See our guides to 304 and 316 stainless steel and duplex 2205.

Surface finish and staying rust-free

The surface finish has a direct effect on how well stainless resists rust. A smooth, closed finish such as a 2B cold-rolled or a bright annealed BA surface holds little dirt or moisture and lets the passive film stay even, so it resists staining far better than a coarse ground or as-welded surface, where micro-crevices trap chloride and grime. Electropolishing takes this further, dissolving a thin surface layer to leave an ultra-smooth, iron-free finish used in pharmaceutical and food work. As a rule, the finer the finish and the cleaner the surface is kept, the longer stainless stays bright, which is why architectural and coastal stainless is specified with a defined finish rather than left rough.

Galvanic and bimetallic corrosion

Stainless can also cause or suffer corrosion in contact with a dissimilar metal. Stainless is relatively noble, so when it is wet-coupled to a less noble metal such as carbon steel, aluminium or zinc, it accelerates corrosion of that other metal while itself staying protected. Carbon steel fixings in a stainless assembly, or stainless fixings driven into aluminium, are the classic cases. The defence is the same as for any galvanic pair: isolate the metals with washers or sleeves where they meet in a wet joint, choose compatible fasteners, and keep the junction dry. A stainless surface left touching carbon steel in a damp joint also picks up iron contamination, so the two problems often appear together.

Stainless steel from ULAMEX

ULAMEX has supplied stainless steel since 1988, in the grades and finishes that suit the environment.

Worried about corrosion in a demanding environment? Tell ULAMEX the application and the exposure and the desk will advise on the grade and finish and return a non-binding quotation. See our guide to welding stainless steel and our supply capabilities, or contact the export desk at [email protected] or +48 504 424 761.

Frequently asked questions