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.

Staining

Identification

This type of problem is quite common and SRL receive a lot of 'phone calls about this kind of problem. The root cause of the problem is as follows... In a nut shell the problem has occurred as a result of a failed pickling attempt, usually someone has tried to pickle the surface or weld zone with a jar of pickling paste and then not completed the task by either not rinsing the area or indeed rinsing the area but not sufficiently hence the appearance of stains that appear as streaks running down the job.

It is important to note that the powerful etching process at work during pickling can only be described as an accelerated corrosion process. Uniform levels of stock is dissolved (etched) from the surface during the contact time. As a result large amounts of metal salts are released into the chemical solution and will be removed during rinsing, however, a large amount of salts will be deposited on the surfaces as Cr , Ni, and Fe salts other by products will be present depending on the steel grade and the make - up of the acid, Fluoride salts will be present also.

You can imaging that a typical 300 series (316L) grade of steel has an analysis of 17%Cr - 12%Ni - 3%Mo and Fe (iron) balance. From this information we can see that the biggest element (salt) to be produced will be Fe (iron), somewhere in the order of 70% of the metals being pulled of the surface will be Fe (iron). If this by product is not rinsed from the surface properly then it will oxidise and manifest as brown rusting within days or even hours.

Possible Detriment

 Apart from the fact that the surface will look unsightly and indeed, if a food/pharmaceutical bearing surface it will look unhygienic. 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 corrosion may occur at the site and lead to greater problem.

Solution

If this problem is spotted early then it is not a great problem since the staining is only superficial and can be removed with a good stainless steel surface cleaner. The trick is to dissolve and take into solution the offending rust and swill it from the surface. In order for the surface to be fully restored to its optimum corrosion resistant properties the "offending" stain needs to be removed since it is a chinch in the surface thus lowering its corrosion resistant properties and inviting corrosion to set-in.

It is important to note that HP washing with (hot water if possible) water should be used for washing down surfaces since rinsing alone is not powerful enough to remove / swill "metals" from the surface.

It may be a good idea (belt & braces) to finish the cleaning operation with a wash down with Enerjex 610 or Enerjex 630.

Note: Owing to the fact that the pickling acid has been allowed to dry on the surface indelible stains may remain etched onto the surface, in this case to reduce the appearance of etching/staining a full blown pickling operation may be required.

Method: To remove the initial staining I would recommend that the offending surface is cleaned with one of the cleaning products listed below (Enerjex 410). A pickling operation may be necessary if the contaminate is severely stained and adhered to the surface. In which case look to the xxxxxxx section for guidance on method and Enerjex products to use.Following the cleaning operation, particularly if ample rinsing with plenty of water is not possible, say indoors or where the general public are, then we would recommend a further rinse or wipe down with a good chelating agent and passivation agent such as StainRest 967C4

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.