Rust and Stains on Stainless Sheet and Plate Products

Stainless steels are, unfortunately, not always stainless. Although we strive to provide products that are clean, rust free and stain free, this is not always possible to achieve.  There are a range of reasons why stainless sheet and plate don’t always have a pristine appearance.

Some grades of stainless steel will form light rust when exposed to mild atmospheric conditions.  Most 400 series stainless steels fall into this category and it is particularly true of type 410 which only contains about 11% chromium.  The martensitic PH stainless grades like 17-4 and 15-5 will also rust.  Note that Alloy 36 (36% NI, balance iron) will readily rust.  It is not a stainless steel, since it has no chromium.

These alloys are commonly delivered from the mill with a light rust or dark oxide on the surface.  Oxide scale from annealing these grades is often removed by blasting with steel shot.  Since these grades cannot be pickled safely using standard nitric-hydrofluoric acid mixtures, it is not unusual for them to not be pickled at all.  If pickling is performed, it needs to be done in a straight nitric only bath which can also leave a very thin layer of iron oxide on the surface.  Any oxide or scale residue will ultimately convert to rust in contact with moisture from the atmosphere.  While this may be a cosmetic issue for some applications, there is no harm or detrimental aspect relative to the performance of the alloy.

Rust

A similar problem arises from time to time with water stains. These can appear on any stainless grade including the higher nickel alloys.  It is more common on sheet products derived from coil and is believed to be related to small amounts of iron dissolved in pickling line rinse water.   If the coils are wrapped while still wet, stains can develop that will appear as various shades of rust color.  This condition is seen less often on plate mill plate products, but if the plates are stacked before they are permitted to dry, it can occur. These stains are difficult to remove and must be removed by mechanical means (i.e. polishing) or chemical cleaning.  Since they are not the result of alloy depletion of the base metal (not corrosion), they typically pose little risk to harming the overall corrosion resistance of the alloy.  However, if the material is going into service in an environment for which it offers borderline corrosion resistance, removal of the stain is suggested.

Localized indications of rust or dirt can also develop from handling or transport.  These indications can also be removed using mechanical or chemical cleaning methods.  Commercial pickling pastes can be used to clean large or small areas.  Citric acid based solutions are also effective for lighter stains or for removal of mild carbon contamination.  Deck washes that contain oxalic acid have also been used.  For tougher stains, brushing with a bristle or stainless brush may improve effectiveness.  Nitric, nitric-HF or phosphoric acid solutions can also be used for more difficult stains, but the economics involved with disposal of the cleaning solution often makes mechanical cleaning more cost effective.