Chemical surface treatments of stainless steel

Successfully using stainless steel depends on environment, grade selected, surface finish, the expectations of the customer and the maintenance specified. Stainless steels provide robust solutions, but in harsh or borderline environments with high expectations for durability, surface finish will have a substantial impact on performance. Surface finishes can be applied mechanically (usually with abrasives) and also chemically.

Understanding how chemical and mechanical treatments will affect the characteristics of the surface will enable the best possible outcome for the client and the structure. Chemical treatment can be used to improve the corrosion performance of the steel.

Stainless steels resist corrosion best if they are clean and smooth. Clean means being free of contaminants on or in the surface that can either react with the steel (like carbon steel or salt) or that create crevices or other initiation points where corrosion can start.

Smooth means having a low surface area at the "˜micro' level. Mechanically abrading the surface can roughen the steel's surface and may also embed unwanted particles.

The common feature of chemical treatments is that they all clean the surface of the steel. They may also smooth or roughen the steel surface, or leave it unaffected depending on which process is chosen. But if carried out properly, they all increase the corrosion resistance.

Stainless steel products

During steel making, sulfur in the steel is controlled to very low levels. But even at these levels sulfide particles are left in the steel, and can become points of corrosion attack. This imperfection can be improved greatly by chemical surface treatment.

Most bar products will be slightly higher in sulfur when produced, so chemical treatment to remove inclusions in the surface of these products becomes more important.

Generally mill finishes for flat products (sheet, plate and strip) will be smoother as their thickness decreases.

New surfaces will be created during fabrication processes, (eg, cutting, bending, welding and polishing). The corrosion performance of the new surfaces will generally be lower than the mill supplied product because the surface is rougher, or sulfide inclusions sitting just under the surface have been exposed or mild steel tooling contamination may have occurred.

Chemical treatments correctly performed can clean the surface and ensure the best possible corrosion performance.

Chemical surface treatments can be grouped into four categories:

Pickling — acids that remove impurities (including high temperature scale from welding or heat treatment) and etch the steel surface. Pickling means some of the stainless steel surface is removed.

Other treatment methods include:

Passivation — oxidising acids or chemicals which remove impurities and enhance the chromium level on the surface.

Chelating agents are chemicals that can remove surface contaminants.

Electropolishing — electrochemical treatments that remove impurities and have the added beneficial effect of smoothing and brightening the surfaces.

Pickling

Mixtures of hydrofluoric (HF) and nitric acid are the most common and are generally the most effective. Acids are available as a bath, a gel or a paste.

Commercially available mixtures contain up to about 25 per cent nitric acid and 8 per cent hydrofluoric acid. These chemicals etch the stainless steel, which can roughen and dull the surface.

Care is required with all these chemicals because of both occupational health and safety and environmental considerations.

Passivation

Nitric acid is most commonly used for this purpose. Passivation treatments are available as a bath, a gel or a paste. Available formulations contain up to about 50 per cent nitric acid and may also contain other oxidisers such as sodium dichromate.

Used correctly, a nitric acid treatment should not affect the appearance of the steel although mirror polished surfaces should be tested first.

Passivation works by dissolving any carbon steel contamination from the surface of the stainless steel, and by dissolving out sulfide inclusions breaking the surface. Nitric acid may also enrich the proportion of chromium at the surface — some chelants are also claimed to do this.

Chelants

Chelants have chemical "˜claws' designed to selectively clean the surface.

The carboxylic acid group COOH is the basis for many chelants, which are used in cleaners, water softening and lubricants.

The pH and temperature must be correct for the chelant to do its job. Turbulent rinsing of pipes and vessels afterwards is important. Cleaning by chelating agents tends to be based on proprietary knowledge and systems, and is less standardised than the other methods described. The successful use of these systems needs to be established on a case by case basis.

Electro-polishing

Most commonly phosphoric and sulfuric acids are used in conjunction with a high current density to clean and smooth (by metal removal) the surface of the steel.

The process preferentially attacks peaks and rounds valleys on the surface and raises the proportion of chromium at the surface.

The technique can have substantial effect on the appearance increasing lustre and brightness while only changing the measured roughness by about 30 per cent.

Precautions

For chemical processes that etch the stainless steel, reaction times will increase with increasing grade.

More care is required with "˜free machining' grades and these will usually require substantially less aggressive chemicals. The sulfur addition in these steels makes them readily attacked by chemical treatments. Care is also required when treating martensitic or low chromium ferritic stainless steels.

Dirt and grease will mask the surface from treatments outlined above. Therefore, the steel surfaces must be free of these agents before applying chemical treatments.

Chemical treatments are useful tools in cost-effectively achieving peak performance with stainless steels.

Australian Stainless Steel Development Association

Level 15, 215 Adelaide Street, Brisbane 4000

For detailed information on this item enter C092 on your enquiry card or use SpeedEmail.