Passivation

Stainless steel is not inert. It carries a chromium-rich oxide film that forms on its own as soon as a clean surface meets oxygen, and that film is what the alloy's corrosion resistance actually consists of. The British Stainless Steel Association puts the film at 1 to 5 nanometres thick, strongly adherent and chemically stable, and notes that it normally repairs itself where damage exposes fresh metal.

Take the oxygen away, or attack the film chemically, and the repair stops. Grundfos states the same mechanism in the CR, CRI, CRN data booklet, in the plainest terms a pump manufacturer could use: aggressive liquids may attack or dissolve the protective oxide film of the stainless steel and thus cause corrosion. The same booklet names the factors that decide whether a given liquid will do it, and lists them as chloride content, pH value, temperature, and content of chemicals and oils. Those four are what a materials decision gets made against, and that argument belongs on corrosion resistance.

ASTM A380 is the practice document behind the word, and it is unusually candid that the industry uses it for more than one thing. It records three meanings. The first is spontaneous: stainless steel forms a chemically resistant surface by itself when exposed to air or another oxygen-containing environment, provided the surface has been thoroughly cleaned or descaled. The second is the removal of exogenous iron or iron compounds from the surface by chemical dissolution. That usually means an acid which lifts the contamination without significantly affecting the steel underneath, and the standard treats it as the primary meaning unless a specification says otherwise. The third is treatment with a mild oxidant such as a nitric acid solution to encourage the film to form.

None of those three is a material property, and that is the practical consequence. "Passivated" does not belong in a materials column next to the grade designation. It describes an operation performed on a part at a point in time, and what it produces is a clean surface rather than a different alloy. If you are comparing two pumps and one datasheet says passivated, you have learned something about how that part was finished and nothing about how it will behave in your medium.

Free iron is the usual reason a passivation treatment gets specified. Carbon steel tooling, grinding wheels, wire brushes and contact with mild steel during handling all leave iron on a stainless surface, and iron rusts wherever it sits, whatever it is sitting on. The rust that appears is not the stainless failing. It is the contamination failing, on top of a sound substrate, and it will keep appearing until the iron comes off.

Welding does something worse than contaminate. The British Stainless Steel Association describes heat tinting as a thickening of the oxide layer, formed by drawing chromium out from below the surface, which leaves the metal just underneath with a lower chromium level. In low-temperature aqueous environments that local reduction in sub-surface chromium can affect the corrosion resistance of the steel. Colour grades the severity. For potable water fabrication, BSSA cites the requirement that all weld heat tints deeper in colour than a pale yellow are to be removed by mechanical dressing followed by acid pickling.

That requirement is also where a specification most often goes wrong, because passivation on its own will not meet it. BSSA is explicit that nitric acid treatments alone cannot be relied on to remove sufficient metal from the surface, so a combination is needed, typically mechanical treatment by grinding or abrading and then nitric acid cleaning. ASTM A380 makes the same point from the other direction, noting that descaling may be necessary before passivation can be effective. An acid dip over weld scale cleans nothing useful.

The two words are not interchangeable, and the trade uses them as though they were.

Ask for the wrong one on a welded stainless fabrication and you will get a clean-looking surface with the chromium-depleted layer still under it.

On a stainless catalogue pump bought as a finished unit, the surface condition was set during manufacture and any certification for it is the manufacturer's to produce. Chasing a passivation certificate for a stock centrifugal is usually the wrong document asked of the wrong party. It also will not answer the question you were really asking, which is whether the wetted materials suit the medium.

Where it does bite is fabricated stainless work, because that is where fresh welds and fresh machining appear. Pipework welded on site, a skid or baseplate modified to fit, a casing repaired by welding, a hopper or a tank connection: all of those leave heat tint and embedded iron on surfaces that are about to see product. If the installation is hygienic, that surface is also the one a cleaning regime has to keep clean, and a rough, tinted or iron-contaminated weld is where a clean-in-place cycle stops being effective.

It is also worth asking whether you need it at all. BSSA's own guidance on passivation says treatments are sometimes specified when it would be better to consider first whether that is strictly necessary, and on a duty where the stainless will self-passivate in service, it usually is not.

Materials are the fourth question we ask when picking a duty, after flow rate, the pipework and viscosity. The question is whether the duty is hygienic, because that is what sets the materials, and temperature follows immediately after it because temperature and the medium together drive the sealing arrangement. Passivation belongs to that fourth answer, as a finishing requirement on fabricated stainless, not as a substitute for it. If you are specifying a hygienic installation, put the standard and its acceptance test on the order for whoever does the welding, and read the hygienic pump standards guide for what else that installation has to satisfy.

Passivation will not upgrade a grade. If the medium is dissolving the passive layer faster than it can re-form, the film is doing exactly what the metallurgy says it should and the answer is a different wetted path, not another acid dip. Grundfos draws that line across its own range rather than blurring it. The CR and CRI are described as suitable for non-corrosive liquids, for transfer, circulation and pressure boosting of clean cold or hot water. The CRN is the one described as suitable for industrial liquids, in systems where every part in contact with the liquid must be high-grade stainless steel. Same hydraulics, different metal, and the choice is made on the medium.

It is also not permanent in the way a coating is. The treatment restores the film once, at the point of manufacture or fabrication. What happens to that film afterwards is decided by the duty, which is why a corrosive medium is a materials problem before it is a pump problem. Send us the medium, its concentration, its temperature and the duty point, and we will work back from there. If the medium is what is doing the damage, start at pumps for corrosive media.