Why stainless steel rusts and what to do about it

It is broken by chlorides, stagnant water without oxygen, weld scale, deposits and particles of ordinary steel. That is why rust on stainless steel is not "bad metal" but a signal about the medium or the condition of the surface. Below are the causes, what to do step by step and when you cannot do without another grade.

Why stainless steel rusts

On the surface of stainless steel there is an oxide layer that shields the metal from the medium. As long as the layer is intact and oxygen reaches it, there is no corrosion even in humid air.

Problems begin in two cases. The first — the film is destroyed locally, usually by chloride. The second — there are no conditions for recovery: no oxygen, there is stagnation, deposits or contamination. Then corrosion begins at the point of damage and goes deep into the metal.

Typical signs:

• red dots and spots that do not wash off with water;
• rusty streaks under a part or along a weld;
• dark or iridescent colours on a weld — traces of heating and scale;
• ulcers and pinholes in places of stagnation and in gaps.

Important: rusty streaks do not always mean that the stainless steel itself is corroding. The red colour is often given by particles of ordinary steel that have landed on the surface.

Main causes

Cause	How it looks	What to do
Chlorides: salt, sea water, chlorine	dots, ulcers, pitting	change the grade to 316, remove stagnation
Stagnant water, gaps, deposits	corrosion under dirt and in crevices	dismantle the assembly, wash, ensure flow
Foreign iron: swarf, steel brush	rusty streaks on top	clean, passivate
Weld scale and chromium depletion	corrosion along the weld	clean the weld, passivate
Acids and aggressive chemicals	darkening, etching of the surface	select a grade for the medium
High temperature	temper colours, scale	remove the heating or take a heat-resistant grade

Let us look in more detail.

Chlorides. The chloride ion breaks through the passive film at individual points and starts pitting corrosion. The sources are sea water, salt, de-icing reagents, chlorine in swimming pool water, detergents. The higher the temperature and concentration, the faster pitting develops.

Stagnation and gaps. Without access to oxygen the film does not recover. Crevices, welded joints, places under gaskets, puddles on horizontal surfaces and layers of dirt — all these are risk zones even for 316.

Foreign iron. Swarf from cutting ordinary steel, a steel brush, iron dust from a neighbouring area grow into the surface. In air the particle rusts, and the red trace spreads over the stainless steel. Externally it looks like corrosion of the part itself, although the metal is intact.

Welding. In the weld zone the metal is heated, chromium is partly bound into carbides, and resistance drops. If the weld is not cleaned and passivated, corrosion begins exactly there. For welded structures, low-carbon grades or steels with titanium are taken.

Acids. Strong acids etch the surface: it darkens, changes colour, loses its protection. Hydrochloric acid and chloride media are not suitable for 304 and 316; for sulphuric and phosphoric acids the grade is selected separately.

What to do if stainless steel has rusted

The order of actions depends on the source. Start with the simplest.

1. Identify the cause. Look at exactly where the rust has appeared: over the whole sheet, along the weld, under dirt, in a gap. The treatment depends on this.
2. Remove the source. Wash off salt and reagents, remove deposits, dismantle the assembly with gaps, ensure drainage and flow.
3. Clean the surface. Use a brush made of stainless steel, not ordinary steel, and a separate tool that has not been in contact with carbon steel.
4. Wash. Wash off the contamination and the residues of cleaning with clean water.
5. Passivate. Treatment with acid removes foreign iron and restores the film's ability to recover. The compositions and the procedure are described in the analysis of passivation of stainless steel.
6. Dry and protect. Remove puddles, ensure ventilation, apply a protective compound if the assembly works in an aggressive medium.

A subtlety: do not clean rust with a disc that has previously been used on ordinary steel. That way you will introduce new iron and get the same streaks in a week.

When changing the grade will help

Sometimes cleaning helps only for a short time, because the matter is not the surface but the grade chosen.

• 304 in a chloride medium. If the part works with salt, sea water or chlorine, 304 is changed to 316 or 316L. The difference between the grades is analysed in the article AISI 304 or 316.
• A welded assembly without heat treatment. Ordinary 304 is changed to a low-carbon one or to steel with titanium. The general principles are in the analysis of welding stainless steel.
• A swimming pool and chlorinated water. Here 316L works, and sometimes more resistant grades too. More details — in the article about stainless steel for a swimming pool.
• Hot media with chlorides. One moves to duplex or super-austenite, calculating the concentration and temperature.

Changing the grade will not help if the cause is stagnation, gaps or foreign iron. First remove the cause, then change the material.

How to prevent corrosion

Prevention is cheaper than repair. The basic rules are simple.

• Ensure drainage and flow: water must not linger on the surface or in crevices.
• Do not leave stainless steel in contact with ordinary steel in a damp zone.
• Use separate tools for stainless steel: brushes, discs, clamps.
• Clean and passivate welds.
• Wash surfaces after contact with salt, reagents and aggressive chemicals.
• Select the grade for the medium, not by the appearance of the part.
• Store rolled steel dry and with ventilation, not under a wet film.

Frequently asked questions

Does AISI 304 rust

Yes, under certain conditions. 304 withstands the atmosphere and fresh water, but corrodes in a chloride medium, under stagnation, at welds without cleaning and on contact with ordinary steel.

Why did stainless steel rust after welding

Because in the heating zone chromium is partly bound by carbides, and scale remains on the surface. The resistance of the weld is lower than that of the base metal. Cleaning, passivation and welding with low-carbon grades help.

What to do if stainless steel has darkened from acid

Wash with clean water, assess the depth of etching and restore the surface. If the darkening is superficial, cleaning and passivation help. If the metal has suffered deeply, the part will have to be replaced, and the grade selected for the specific acid.

How to remove rust from stainless steel

Remove the source, clean the surface with a stainless steel brush, wash and passivate. Abrasives used on ordinary steel must not be used — they introduce iron and cause new streaks.

Does stainless steel rust in a swimming pool

Yes, if it is 304: chlorine and salt destroy the passive film, especially in gaps and on welds. For swimming pools, 316L is taken.

Can stainless steel rust from ordinary steel

Yes. Particles of carbon steel that land on the surface with swarf, a brush or dust rust themselves and give rusty streaks. Such a trace is often taken for corrosion of the stainless steel, although the metal under it is intact.

Does citric acid help remove rust

With a weak solution you can remove surface traces of foreign iron, but this is not full passivation. For critical assemblies, proven compositions and the holding time according to the instructions are used.

Place an order for rolled steel with the required resistance

If the corrosion is caused by the medium and not by maintenance, the matter is the grade. Choose a position in the exchange catalogue for your conditions and specify the grade, size and surface in the order.

• Catalogue of stainless steel products
• Passivation and pickling of stainless steel
• Stainless steel for a swimming pool