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.

Burn Through   and Heat Treatment

Identification

Burn Through and Heat Treatment

This is also a very common and unavoidable feature. It is attributed to the exposure of the steel surface to temperatures above approx. 400°C. When the stainless steel surface is raised to around this temperature it tends to oxidise in the atmosphere. The degree of oxidation will depend on the steel thickness, temperature reached and to an extent the analysis of the material.

Possible Detriment

Apart from the fact that the surface will look unsightly and indeed, if a food/pharmaceutical bearing surface it will look unhygienic. The product will detach itself from the surface very easily and will readily contaminate any product that will pass over its surface; therefore it must be removed before the fabrication enters into service.

The surface of the steel is also contaminated and will not form a passive film therefore is susceptible to corrosion attack. Since this area is of the same analysis as the parent material and not a weld deposit then if the surface is mechanically polished and then subjected to a pickling operation the said area will not reappear as an unsightly surface defect. See "weld etch" example in trouble shoot section.

Solution

A very common sight is to see these areas removed by mechanically polishing over them with a grinder with a course grit abrasive, this is a bad move since you are exchanging one problem for another, often to a worse condition since a scared surfaces often as a result of mechanical polishing are more prone to corrosion attack than the original condition (burn-through). The way to deal with this condition is to etch or pickle the oxide away thus minimising the amount of work on the surface and minimising the risk of further damage. If the surface is to be mechanically polished as a design requirement then this is best polished after all the welding is done...if the fabrication requires totally pickling, the areas (burn-through) under the polished surface will not reappear as unsightly surface defects since they are of the same microstructure. If the fabricator insists on spot mechanical polishing the burn-through then the area should be finished with a minimum of 180 grit (0.6Ra µm) or better.

Method:
 If the fabrication has been mechanically polished to a particular grit finish then it will most likely have traces of grit compound and fat from the polishing operation these will inhibit the pickling process and even result in uneven pickling / etching and staining. To avoid these possible problems first pre-clean the entire fabrication with one of the cleaners listed listed in the product section. The fabrication can then be pickled, whether by immersion or manual method.

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.