Stainless  Restora​tion Ltd

Metal Finishing and Surface Engineering Technologies

SRL Technical Library

PICKLING AND PASSIVATION TROUBLESHOOTING  During, following or indeed before the cleaning operation all sorts of strange things can occur as a result of the chemicals used. Our technical library gives a variety of examples that SRL has encountered along its way. This list is by no means complete - these pages will be continually updated as we/you experience other strange phenomena.

Pitting

Identification

Type of corrosion. Pitting is a severe type of surface corrosion where attack is concentrated in a single area or pit.

In the picture shown pitting is severe and many pits are located in one area. The cause of the pitting in this case was due to "grinding" of the surface with course grit abrasives thus leaving deep scratches that have become sites for corrosion attack. The service conditions were aggressive therefore the pitting/corrosion started easily. It is often thought that this type of corrosion starts when a grinding disk that is used on a mild steel surface is then used on a stainless steel surface, this is very seldom the case. The fact that the "skin" or mill finish, in this case the cold rolled smooth surface finish, was broken and rendered rough or course by the grinding operation. The brown matter present is iron oxide or "rust" this is the by-product of the corrosion process.

Possible Detriment

If the surface is a product contact surface then the "rust" or iron oxide will contaminate the product. If the product or medium passing over the area is aggressive enough then further corrosion will occur at the corrosion site and lead to greater problem, even mechanical failure.

Solution

In order for the surface to be fully restored to its optimum corrosion resistant properties the "offending" scare / scratch needs to be removed since it is a chinch in the surface thus lowering its corrosion resistant properties and inviting corrosion to set-in.

Once the scare/scratch or surface defect is removed (total removal is not necessary, blending the surface area into the surrounding area by use of a mechanical polishing operation will suffice, ensuring to polish not grind leaving a surface of 180 grit minimum.

Always finish with a chemical cleaning operation to ensure all "free iron" and iron oxide is removed and the passive film of the stainless steel is restored.

Note: There is documented evidence that the corrosion resistance of a given surface is a function of its smoothness, cleanliness and chemical analysis of its microstructure.

If a chemical cleaning operation alone is used then this may serve only as a temporary fix, if the service conditions are aggressive enough the problem will return.

Method: In this type of case the corrosion is usually deep therefore a proper pickling operation needs to be conducted. The pickling can be done locally (by manual application) or the whole item may be pickled by an immersion or spray pickle application).

Products  Available

Pickling Products

Cleaning Products

Technical  Notes

It is important to follow the instruction given for each individual product used and the advice given in the "product application data sheets" available on line in the product section of SRL Chemical and Analytical Services Ltd.

Information given in this page may be subject to alteration without notice.

Care has been taken to ensure that the contents of this publication are accurate but SRL Site Services Ltd do not accept responsibility for errors or for information that is found to be misleading.

Suggestions for, or descriptions of, the end use or application of products or methods of working are for information only and the company accept no liability in respect thereof.

Before using products supplied or manufactured by the company the customer should be satisfied of their suitability.

If further assistance is required, we have access to extensive research facilities, we will often be willing to help.

Stainless Restoration Ltd, Adamsons Industrial Estate, Hyde SK14 1EE

Stainless Restoration Ltd offer an Electropolishing service to the stainless steel fabrication/manufacturing industry at its newly refurbished fully automated Electropolishing plant at Hyde Manchester.