Friday, December 9, 2011

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.



2 comments:

Anonymous said...

Dr. Brooks,
Do you know if re-passivating a piece of pitted stainless is beneficial? Would the stainless form a new passive layer (even possibly over the pits)? I realize you aren’t a metallurgist but thought you might know or who to ask. Thanks
Nathan

John Brooks said...

You are correct, Nathan - I'm not a metallurgist. My reading suggests that there is no simple answer, but I'll discuss this with more qualified colleagues and, of course, readers are welcome to comment.

Some damage to the passive film may heal if there is sufficient access of oxygen and the material contains sufficient molybdenum.

The pit may form an electrochemical cell, propagating the corrosion inside the material.
The pit may become blocked by corrosion products, resulting in a strongly anoxic environment, which will make it impossible for the passive film to re-establish.

If the pitting has resulted from chloride attack, it is important to remove the chloride ions. If contaminants from machining or welding have caused the pits, it is important to remove them too.

Cleaning the surface thoroughly and treating the area with 7M nitric acid for 60 - 80 minutes may stop the corrosion and certainly will do no further harm.

Have a look at Application note 28-9433-77 AA from www.gelifesciences.com

See also the preprint in Journal of Corrosion Science and Engineering, Volume 12 Preprint 5, 2009.
Passivation , Repassivation and Metastable pitting Of Bare and Surface Treated 316L stainless implant In Hank Solution.
Subir Paul and Nisha Prasad.

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