Stainless Steel Passivation

When stainless steel is subjected to any heat process welding or machining the material looses properties within the infra structure of the metal. The chromium content is decreased and positive irons are deposited onto the welded or machined surface. This welded or machined area of the stainless steel will be prone to corrosion (rust). The only way to overcome this problem when working with stainless steel and heat is to passivate the system. There are several theories as to how the stainless steel should be passivated.
The original concept was to firstly degrease the pipe using Potassium Metasilicate or Potassium Hydroxide (Caustic) at a 1% solution to volume. This technique will disperse any grease or foreign bodies from the material. This system is still the best way forward.
The second step is to introduce an acid compound to the material to increase the oxidisation and restore the chromium element of the stainless steel. In the past Nitric acid at 40% volume was considered the only way to perform the passivation of the metal. The problem with this is that Nitric Acid is a hazardous material and requires a permit to dispose of the effluent. This can be expensive and in today’s carbon foot print life style a no no.
A more efficient process is to use Citric acid. This process is more environmentally friendly and more efficient in the passivation process. The normal percentage to volume is maximum 15%. The disposal of the effluent is to drain, this will decrease costs and work with any Environmental company policy. The final process is to determine if the passivation has worked. There are several processes that can be used. Using a salt spray onto the material and left for 24 hours, brown rust marks will appear if positive irons are present. Ferroxyle test, this is not advised on food and beverage processes and pharmaceutical, as the chemical when mixed will create cyanide. In the past we have introduced a spool piece within the system to be passivated with the major pipework. This spool piece is submitted to the ferroxyle test and disposed of.
HIPS undertake stainless steel passivation in house, we have a purpose built skid unit which can perform the tasks of passivation of the pipework, and smaller spool pieces can be dipped. Full certification will be supplied for all passivation up to FDA standards