If you’re new to welding, you may notice that your welds aren’t coming out as clean as you hoped. You may see a dark or black mark around the weld, or at the very best rainbow-like scale, which is probably not what you were wanting to see.
Discolouration and other surface imperfections caused by welding aren’t just cosmetic issues though. The discoloration is called “heat tint’. It has been caused by the high temperature of welding, and the colour change is due to damage to the stainless steel’s heat-damaged chromium oxide layer. This layer protects the steel from rust, so without it the metal becomes vulnerable to corrosion and surface rust.
It is important to restore the stainless steel anti-corrosion layer, in order to preserve the steel’s strength, shape, and surface, in order to provide a durable and attractive product. This can be accomplished by removing the heat tint and allowing the chromium oxide layer to reform. There are various ways of doing this, using mechanical, chemical or electrochemical methods. These methods all result in a clean and polished metal work piece, but the time and effort involved with each process is very different. Some of these methods also result in surface damage, as we will explain below.
Mechanical Weld Cleaning
Mechanical weld cleaning is commonly used for removing heat-tint from stainless steel workpieces because the tooling is inexpensive. All that is required is to rub the surface with a material harder than stainless steel, to remove the top layer. The choice of actual cleaning tool depends on the requirements of the project and your preference. For example, abrasive products such as sandpapers or flat discs remove the base material and weld splatter, but wire brushes wont. Flap discs are great for blending and finishing the metal surface post-weld to prepare it for paint, primer, or powder coating. Wire brushes are a good option for removing just the heat tint, rust, and mill scale on weld joints that have not been too damaged by the welding process.
However, there are hidden costs with the abrasive method. When the abrasives remove the surface of the stainless steel metal, they cause damage to the surface of the stainless steel and leave microscopic grooves in the metal. Not only does this alter the look of the finished product, but the damage can accelerate corrosion. There is also the risk of contamination. If the abrasives have ever been used with carbon steel, using them on stainless steel embeds the surface with iron particles, which prevents the chromium oxide forming and promotes corrosion.
Mechanical weld cleaning methods are also time-consuming, which could be a disadvantage for many businesses. They require dexterity and skill to effectively clean your welds. Finally, the use of rotating or vibrating machinery can result in health problems for the user. In many regions there are limits on the duration of use of such tools to minimise repetitive strain injuries occurring
These are some of the reasons many welders prefer to use chemical cleaning methods instead.
Chemical Weld Cleaning
The most common product used for chemical weld cleaning is “pickling paste”. It is very effective at removing surface impurities, including rust, stains, and heat tint scale from metals.
Pickling paste may in the first instance seem quite easy to use. All you need to do is apply the paste to the affected areas using a spray or brush, leave it to interact with the metal, remove the product, then neutralize the surface with a neutralizing agent. Unfortunately, the amount of time required for the past to work depends on its concentration and the ambient temperature on site. The “safer” the picking past and the colder the environment the longer the pickling process takes.
It’s important to note that pickling paste contains extremely toxic chemicals, including hydrofluoric and nitric acids. These acids are extremely hazardous and can cause long-term health complications, such as skin damage and acute lung injury if directly exposed to the paste. The PPE required to perform this task is the most comprehensive of all three methods and for good reason.
Because of these hazards, many health experts and regulators are calling for the industry to discontinue the use of pickling paste. The negative health implications is one reason why chemical pickling is generally only used in properly set up industrial workshops. The operator must wear full-body protective equipment to prevent directly touching the paste or inhaling its fumes, and precautions must be in place to prevent the residue from leaking into the environment and causing ecological damage.
Electrochemical Weld Cleaning
Electrochemical weld cleaning may be one of the newer methods of weld cleaning. It is quickly gaining popularity as it becomes recognised as the fastest and safest option among the three weld cleaning methods. For example, the TIG Brush, which pioneered the use of a conductive brush, uses electrochemical cleaning to effectively removes rust, surface impurities, and heat tints. The electrochemical (or electrolytic) cleaning process may also make the metal workpiece smoother, shinier, and brighter – a huge perk for projects where appearance is a priority.
The TIG Brush utilises a conductive weld cleaning brush to apply a mild electrolytic cleaning fluid to welded joints or surfaces. Electric current is used to accelerate the cleaning and passivation process. Additionally, the TIG Brush has been shown to “passivate” the surface of stainless steel, providing a higher resistance to corrosion than other methods. Additional benefits include:
- Compared to mechanical weld cleaning and/or the use of pickling paste, electrochemical cleaning is far less laborious.
- When used with the recommended PPE, electrochemical weld cleaning is much safer for the user and the environment.
- During the weld cleaning process, passivation occurs immediately, a second step is not required to achieve this result.
- You can choose the area of metal to be treated, no need to treat the whole surface.
- It is portable, allowing greater flexibility no matter where it is used.
- Running costs are minimal making it a very cost-effective solution for small to medium businesses.
Choosing the Best Post-Weld Cleaning Method
Among these three weld cleaning methods, electrochemical weld cleaning is the superior option. It’s fast, safe, and effective at cleaning and passivating your metal pieces professionally.
If you’re looking for an easier post-weld cleaning method, check out the TIG Brush. This tool eliminates the need for dangerous substances such as pickling paste and tedious weld cleaning processes, removing surface impurities within minutes. You only need to dip the brush in cleaning fluid then swipe it across the weld to instantly achieve a clean and passivated stainless steel surface.