Passivation of stainless steel with Citric Acid

Many people are not aware of the need to passivate stainless steel to keep it "Stainless".

Why is it needed?

All stainless steel needs to be cleaned and passivated to protect the surface from contaminants. Stainless steel has a self protecting surface that is produced by an oxide layer. If when in manufacture and handling, the stainless comes in contact with carbon steel such as used for cutting, rolling, bending, drilling or whatever processes have been done, there is always some transfer to the stainless steel, and this needs to be removed and the oxide surface retored, that is what the passivation proccess does.

The process used to be done with nitric acid, which is dangerous to handle and has many drawbacks, especially eniromentally.

The latest process uses Citric Acid which is much safer and easy to do and can be handled without the fear of doing damage to yourself or things around you. Citric Acid can be sent through the post because it is classified as "Non Dangerous Goods" and is also environmentally safe.

The process that we use for passivating may be a little different than those that only want to prepare a small number of plates.

The method we use can be copied easily if you have a lot to do.

First the acid powder is mixed with distilled water (one pack mixed with 2 litres of water) or 10% citric acid per volume. If you have more cells to do, just increase the number of packs and also the amount of water.

Next the acid water is placed into a small plastic bowl and a cheap under water aquarium pump is connected to a hose that is connected to the inlet of the dry cell. If you have more than one dry cell to passivate, connect the outlet of the first cell to the inlet of the second cell and then return the outlet from that cell back to the bowl. You can chain many cells together using this method, we do as many as 12 dry cells at a time.

It is best if the acid water has a temperature of around 120F or about 50C. Set the pump going and leave for about 20-30 minutes, all the cells will be processed at the same time. Drain the acid water from the cells and rinse with distilled water. Blow compressed air through the cells.

The cells have now been cleaned and passivated at the same time.

If you only have some loose plates to passivate, same process only just place the plates vertically in a bowl and add the warm acid water to cover the plate. Leave for up to 1 hour and then remove. Rinse with distilled water. (wear plastic or rubber gloves to protect the plates from your oily fingers) Dry with some newspaper or air dry and assemble your cell with care so as not to contaminate the surfaces with either oily hands or by touching with other metal objects.

Pack availabe to make up 2 litres of water.

Another Popular Method

First degrase them with isopropil alcohol, then put plates for one-two hours in HCl (hydrochloryde acid 15-30%). Rinse them with water an wipe with dry cloth. Assemble generator and put citric acid in it. Circulate that acid for 1h and poure out from cell. Rinse with distilled water cell twice and leave the cell for one day on table so that air can go in the cell. You don't have to dismantle cell after citric acid. You are ready to go