The smooth patina of chromium oxide left after treatment with SSWTR is visible on this weldment, which has been half-treated on the left side.
After fabricating stainless steel, machining or welding, restorative treatments are needed to keep your fabrication free of corrosion – pickling and passivating. Unfortunately, stainless steel’s corrosion resistance also makes it resistant to simple finishing treatments. Chemical treatments, particularly, have traditionally been based on using strong and dangerous acids.
Machining stainless steel may leave iron oxide on the surface of the stainless from the iron content of the tools or residuals left from a previous task. Welding will leave a heat or weld tint discoloration on the weld surface and the heat affected zone. In order to maintain the corrosion resistance of stainless steel, it must be pickled to remove the oxides and debris on the surface.
The effects of welding are not limited to the presence of pure melted metals, but also gases, alloying materials and the effects of heat itself. It is a very dynamic place. During the molten phase of welding, contaminants mingle with the chromium in stainless, creating new compounds, including oxides. These oxides gather on the surface of the weld, creating surface contamination, reduced levels of chromium and the well-known weld tint and/or smoky appearance.
Pickling and passivating
Pickling removes the layer of oxides from the weld and heat-affected zone (HAZ). The use of a solution containing hydrofluoric (HF) and nitric acids is a popular choice for pickling stainless steel welds. There are, in fact, many recipes for pickling chemicals. This is the reason for the debate today in the stainless steel fabrication industry. Some of the other chemicals used may contain citric (does not pickle – passivates only), phosphoric and nitric acids, nitric and hydrofluoric acids, or sulfuric acid. And, when the private concoctions that fabricators themselves brew up are added, the list can grow quite long.
What are some of the decisions the fabricator must make to produce and finish a sound fabrication while protecting the operators and our environment? Consider first the steps needed to passivate a stainless steel fabrication.
If the pickling solution contains HF acid, one should remember that HF acid is a dangerous and toxic material. If HF is mistakenly applied to a hot weld, it will produce a poisonous gas which can damage lung and or eye tissue. Diluting HF acid with water or other acids can be dangerous as well, and may contribute to and camouflage dangers that are not readily apparent.
For example, HF acid is not readily washed off of your skin. HF acid may permeate one’s skin as a contact poison, or damage internal tissue. HF acid also will attack the calcium in the body, which can lead to bone degradation and a reduction of calcium in the blood stream, which, in turn, can lead to heart failure. These above concerns are described in many medical and non-medical publications. Please consult them for complete information.
HF acid may also attack the fabrication you are trying to pickle. If HF acid is left on the fabrication too long, thinning of the base material or pitting can occur. Rinsing chemicals containing HF acid with water is common practice in the field. However, chemical manufacturers strongly recommend that HF acid present even in very small percentages should be neutralized before proceeding with rinsing or the passivation process.
Once the stainless steel fabrication is properly pickled, the next step depends on the acid being used. It may require neutralizing the pickling acid, which, again, is a must for products containing even small percentages of HF acid. At the very least, rinsing is required before one applies a passivating chemical.
When passivating stainless steel, most of the industry agrees that, while there are many choices available, nitric acid is very effective in restoring a healthy level of chromium and the corrosion resistant properties on the austenitic materials and other high chrome-nickel alloys. This chromium-rich surface is very thin. After so much work is invested in cutting, welding, pickling, neutralizing and passivating, care must be taken to protect it. Poor housekeeping or shipping procedures can cost thousands of dollars due to field repairs.
Some pickling chemicals cannot be sprayed without wearing a chemical moon suit with a compressed air supply tank in a properly equipped building. This serves to highlight the health, safety and environmental risks to the worker applying the product, as well as to his fellow workers. These aggressive acids add to your operational expense: applying, neutralizing, rinsing, passivating and rinsing again are labor-intensive and time-consuming.
There is a lot to clean up on this tubing weld, both on the bead itself an in the surrounding oxide colors typical of stainless welds.
Praxair’s pickling/passivating treatment cleans up the tints and the other oxides, and passivates the weld for good corrosion protection.
A wish list for a better product
The fabrication industry has long wanted a biodegradable product that works consistently and evenly without risking the safety of the operators or the environment. If we made a wish list, the product would:
• Be a 3-in-1 process: – pickling, neutralizing and passivating in one product
• Require only a water rinse for the fabricated piece when it’s done
• Be applied by dipping, painting or spraying without wearing a chemical suit (though one would still wear the appropriate Personal Protection Equipment [PPE]).
This sounds like an impossible list, but it’s not. We’ve been working to create such a product for years. Finally, after years of testing for chemical safety and effectiveness, a US and Canadian patent was issued for this new 3-in-1 product for treating stainless steel, without the use of HF acid. It’s here, now. And it is biodegradable.
Praxair’s ProStar™ Stainless Steel Weld Tint Remover (SSWTR) paste is designed to remove iron and oxides (pickle) at the same speed as products that are HF acid-based, and while other products require rinsing or neutralizing within a definite timeframe, you can allow the SSWTR to remain on the fabrication for hours without the risk of thinning or pitting. You are free to remove the SSWTR when convenient with water and without a neutralizer. Further, allowing the SSWTR to remain on the part restores a chromium rich surface. In other words, it also passivates the welded piece.
When complete, all one needs to do is to rinse the stainless steel fabrication following the local codes. Generally, this means using a clarification sump to collect and separate the metallic oxides and particles from the runoff water. Be sure to review these procedures with your local government requirements and authorities, which may vary from region to region.
SSWTR can be sprayed over the entire length of a large fabrication, which reduces the cost of transporting to a dipping site. If dipping is your preferred method, no problem! SSWTR is also available as a very stable and nearly odor free dipping gel with virtually zero adhesion to the part. This greatly reduces acid adhesion rates, evaporation and the ventilation requirements you would have with the more aggressive acids.
SSWTR produces an even patina of chromium oxide over the entire area treated. This provides a visual indication of the healthy chrome layer being produced; gone are the water marks from the uneven application of aggressive acids, which can result in products being shipped back to be re-passivated, creating additional expenses for the fabricator.
There are other advantages. If you’re going to use this new product, speak with your insurance company about reducing your insurance premiums, because you had the foresight to improve your fabrication by making it a safer place to work. It certainly is worth asking for.
The use of SSWTR has been tested for the last five years by two large engineering companies, and it has been used by dozens of smaller fabricators with excellent success. The results have won their praise, resulting in:
• Improved safety for workers
• Reduced impact upon the environment
• Better results for the fabricator because of consistency and safety
In an industry defined by technical skills, large and expensive equipment and costly materials, it’s an eye-opener to consider how much one’s operating costs may be influenced by a change from one proprietary chemical treatment to a much simpler one. But add up the costs of your passivating and pickling costs as they are now. And then think about how much less they could be if you could replace them with a one-step, safe and effective replacement. If you weld stainless steel, it’s worth a close look.