It's the pits - bacterial hideaways

Modern food processing is often carried out in stainless steel equipment - tanks, pipes, valves and conveyors are commonly made of various grades of stainless steel.  We tend to think of "stainless" as not suffering from corrosion.  To a large extent, this is true.  Stainless steel has a natural oxide coating that prevents water molecules from oxidising the iron.

However, stainless steel can still corrode where grain boundaries or embedded contaminants allow water to access the iron.  The contaminants might be grinding swarf from welding or repairs.  Stainless steels may therefore benefit from a process called passivation, in which the surface is cleaned with sodium hydroxide and then treated with nitric acid.  This restores the oxide film.

We use stainless steel in our laboratory experiments and routinely passivate with hot nitric acid.  One of my students used a bottle labelled "Concentrated nitric acid" from the chemistry laboratory to passivate some new samples.  Unfortunately, it appears that the contents were actually Aqua Regia, a mixture of nitric and hydrochloric acids.  (How often have I said that correct labelling is critical in food safety?).  

Chlorine ions are extremely electronegative and react strongly with certain compounds.  They can severely damage stainless steel.

The first photograph shows two coupons treated with the acid mixture.  It is obvious, even to the unaided eye, that the surface is pitted.  Chloride pitting tends to occur at right angles to the surface, so deep pits form rapidly.  Obviously, the use of aqua regia is a very extreme case of chloride attack, but even food materials containing sodium chloride will eventually attack stainless steel.  Even 316 stainless steel, which contains molybdenum that helps to stabilise the passive film, will corrode if exposed to high levels of chloride ion, or if the oxygen level is very low.  This is what may happen under a biofilm, where the bacteria use up the oxygen.  The area then becomes anodic and current flows, resulting in corrosion and the formation of a pit.

I took a couple of coupons to Dr. Jen Wilkinson who runs our Scanning Electron Microscope.  She took the following images, which show clearly the damage to the surface and the deep pits caused by the corrosion.  The second image below shows the interior of the pit.  Bacteria could easily enter the pit and would be very difficult to remove during cleaning.  If the bacteria form a biofilm, they will be protected by the extracellular polymeric substances (EPS) which glue them to the surface and may inactivate disinfectants.  The bacteria will be impossible to remove.