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How To Clean Metal Before Welding
In the world of metal fabrication and welding, achieving a clean, pristine surface on your workpiece is paramount for strong, durable, and aesthetically pleasing welds. However, attaining solid and clean welds is more than just welding it correctly. To get there, besides the correct settings and years of experience, you'll also have to consider proper weld preparation.
That's why, in this comprehensive guide, we'll delve into the essential techniques for cleaning metal surfaces before welding. From removing stubborn galvanized coatings that can pose health risks when heated to eliminating layers of paint, rust, and tenacious mill scale, we'll walk you through each step of how to clean metal before welding.
Source: https://www.thefabricator.com/thefabricator/article/cuttingweldprep/5-steps-for-proper-weld-preparation
Can You Weld Through Paint, Rust, Dirt, Or Zinc Coating?
Theoretically, you can weld through surface contaminants such as paint, rust, or zinc coating. Due to high penetration, Stick welding with E6010 rods or flame (oxyacetylene) welding will quickly burn through layers of rust or coating. However, welders will rarely encourage you to do so. This approach can be used in specific and relatively limited situations where pre-weld cleaning isn't cost-efficient. Otherwise, welding painted, rusty, or galvanized metal can cause defects and issues.
First of all, welding through contaminants can compromise the quality of the weld. Paint, rust, and dirt can introduce impurities into the weld, weakening its structural integrity. Loss of structural integrity leads to defects such as cracking or entire weldment failure. Coatings and contaminants can also interfere with the penetration into the base metal, creating weak spots that make liability in the weld.
Additionally, welding through paint or zinc coatings can pose serious health risks to the welder. Once the zinc or paint is heated, it starts to boil, producing highly toxic fumes that can present serious health risks to welders. OSHA Regulation 1926.354 for employees states that in enclosed spaces, all surfaces covered with toxic preservatives must be stripped of all toxic coatings for a distance of at least 4 inches from the area of heat application. Otherwise, the employees must be protected by air line respirators.
Finally, welding through dirt, paint, or other coatings can produce a messy, uneven appearance. Boiling paint also makes spatter that flies around, creating aesthetically unpleasing welds and fire hazards. So, the more time spent preparing the weld will decrease the time spent on post-weld cleaning and reworking.
What is Galvanized Coating, And Why Do You Have To Remove it Before Welding
Galvanized coating, often called galvanization or simply "galvanizing," is a protective surface treatment applied to steel or iron objects to prevent corrosion. Galvanization is a layer of zinc implemented onto the metal surface, and it is spread through various methods, typically by dipping the metal into a bath of molten zinc or by using a process known as electroplating.
The purpose of galvanizing is to create a barrier between the base metal (usually steel or iron) and the surrounding environment. Zinc is a typical choice due to its excellent corrosion-resistant properties. When exposed to moisture and oxygen, zinc forms a protective layer of zinc oxide on its surface, which acts as a sacrificial barrier, preventing the underlying metal from corroding.
Photo by @thekidthattigss (TikTok)
However, while galvanizing provides excellent corrosion protection, it can pose health risks if the galvanized metal is heated, such as during welding.
Risks Of Welding Galvanized Steel
Welding galvanized metals can pose several health and safety risks due to the zinc fumes that form during welding. When galvanized metal is heated during welding, zinc on the surface can vaporize and form zinc oxide fumes. Inhaling larger amounts of these fumes can lead to a condition known as "metal fume fever." Long-term exposure can cause more severe illnesses or even lung cancer, so it is essential to reduce exposure.
Welding outdoors will significantly reduce the risks, but the best practice is to remove the galvanized coating or paint whenever possible. Even Occupational Safety And Hazards regulations call for removing a toxic layer at least 4 inches from the heat source, or in our case, the weld.
How to Remove Galvanized Paint Or Coating
Removing and cleaning galvanized coating and paint before welding is essential to ensure a safe and effective welding process. However, due to the nature of the zinc, even the removal process should be handled with care, including proper precautions, the right tools, and Personal Protective Equipment (PPE).
Zinc is a helpful trace mineral that benefits our body, but excessive exposure can cause various health risks. That's why you should wear protective equipment and protect your eyes and skin from debris before removing the zinc coating. Therefore, make sure you wear gloves and face and hand protection, and to remove galvanized paint or coating, you can use the following methods:
- Brushing: If you are dealing with loose or flaking galvanized paint or coating, you will only need a wire brush. If the galvanized paint is more persistent, you can use power tools with a wire brush attachment. Brushes are also recommended in tight areas where you cannot use power tools or grinders.
- Grinding: Grinding is almost always a better solution for thicker or persistent galvanizing. An angle grinder equipped with an aluminum oxide or silicon carbide abrasive wheel will remove the galvanized layer thoroughly, and you should do it all around the weld joint. However, grinding can be potentially hazardous since zinc dust or particles can fly around the workshop. Also, the abrasives can heat the zinc, producing limited fumes. Additionally, if you grind too deep, you can incorporate zinc into the joint, which can compromise the integrity of the weld. Nonetheless, grinding is a go-to method for zinc-layer removal if you do it right.
- Chemical Treatments: Most acids can remove galvanization and are often used to clean the entire workpiece. Depending on your solution, this process may require multiple applications to remove all galvanizing traces from the metal surface. Weak acids, such as the ones from vinegar or lemon, can remove the zinc coating with little to no risks or irritations, but it will take time. Meanwhile, muriatic acid is a cheap and diluted form of hydrochloric acid. It can remove thick galvanized coatings in a couple of minutes or even less, but it is highly toxic and can damage eyes, skin, or respiratory system. It also releases highly flammable hydrogen, so you should be careful when using it.
- Mechanical Stripping: a simple hammer and chisel can also help you remove zinc coating, and you don't have to worry about debris or toxic solvents. However, this process can be slow and hard, so pneumatic hammers and automated stripping are used in specific applications.
Remember that grinding, brushing, or mechanical stripping can leave some debris or zinc dust, so make sure you clean the workplace before welding. High welding heat can also interact with some acids and dust, so besides welded pieces, ensure your environment is also clean before welding.
Welding Painted Metal
Similar to zinc coating, paint on the surface of the metal can cause several health and weld integrity issues if you don't remove it before welding. Paint acts as a barrier between the welding arc and the base metal, which can lead to a lack of penetration, improper fusion, or overall contamination of the weld.
Paint may contain various additives, pigments, and binders that can release harmful gases and produce slag when subjected to high temperatures during welding. Depending on the type of paint and its composition, toxic fumes, and gases can harm the respiratory system or cause skin and eye irritation.
The presence of paint can also alter the mechanical properties of the base metal, such as its strength and ductility. Paint can boil, creating holes in the weld, known as porosity, which leads to cracking and failure. Also, welding through paint can cause irregularities on the weld surface, making it difficult to achieve a smooth, uniform finish. This can be aesthetically undesirable and may require additional post-welding cleanup.
How To Remove Paint Before Welding
Unlike zinc coating, paint usually isn't so tightly fused with the surface of the base metal, so to remove it before welding, you typically won't need power tools or big guns. Flakey or loose paint will come off with sanding paper, but you can also use a brush and attachments on the power tools. The most effective ways to remove paint before welding are:
- Brushing: As noted, most of the paint can be easily removed with a wire brush. Brushing is a relatively easy and quick operation that will remove the paint from the surface quickly so you can return to welding in no time.
- Sanding: Sanding is another fairly easy and quick procedure that will help you remove the paint from the metal before welding. However, hand sanding can be time-consuming and hard, especially on larger pieces, so most welders use a sanding disc or wire wheel mounted on an angle grinder or other power tools.
- Paint Strippers: Paint strippers are chemicals that can penetrate through multiple layers of paint and weaken the adhesion between it and the surface. You apply them to the paint, let them react, and easily strip the paint with a brush. These can be used on various types of paint and are recommended for thicker and persistent paint layers.
Welding Rusted Metal
Welding rusted metal can present several potential issues and challenges, both in terms of weld quality and the safety of the welding process. Rust compromises the structural integrity of the base metal, making it weaker. Welding on rusted metal can result in welds with reduced strength, which may not meet the required structural or load-bearing specifications.
Rust and corrosion often create uneven surfaces with pits and crevices. These imperfections can trap gases and impurities during welding, leading to incomplete fusion and weaker welds. Once the gas and contaminants are trapped inside the weld, they can cause porosity or incomplete fusion, leading to weaker welds and cracking.
Photo by @im.natelee (TikTok)
Like with paint and zinc, welding through rust can create iron-oxide fumes that can cause metal fume fever. Additionally, rusted metal may produce more spatter during welding due to moisture and contaminants. Spatter can burn your skin, cause a fire in the shop, or require more time spent cleaning after the weld.
How Do You Remove Rust Before Welding?
Due to potential hazards and issues, removing rust from pieces before welding is crucial to ensure a clean and sound weld. Before you start cleaning the rust, you should first evaluate it. Light surface rust typically falls off quickly, while heavy and thick rust might require more attention.
- Removing light, surface rust before welding: rust-colored deposits on the surface of the metal present the first stages of rust or surface rust, which can typically be easily removed. Like with paint, you can use a wire brush or sanding paper or discs to remove it. Naturally, it comes off easily, but removing surface rust can create hazardous rust dust, so make sure you wear a mask.
- Removing heavy, thick rust before welding: in further stages of development, rust slowly penetrates the surface of the base metal and creates thicker layers. On the top of the surface, the rust scale is formed, but that's just the tip of the iceberg. While you can remove the scale with sandpaper or sanding wheels and brushes, the thicker layers require grinding, blasting, or chemicals. Angle grinders with a flap disc or a sanding disc will quickly remove large-scale heavy rust but be aware of the dust and other particles. Acid-like chemicals are also a good solution, but you should handle them carefully. Finally, abrasive blasting or sandblasting is used to clean highly rusty pieces, but its equipment is often reserved for industrial applications.
What Is Mill Scale, And Can You Weld Through It?
Mill scale (also called scale) is the flaky surface that forms on hot-rolled steel. Mill Scale is found on all hot-rolled steel products unless processed in a protective, inert atmosphere. In the metalworking industry, many welders often mistake mill scale for rust. However, the mill scale is formed at elevated temperatures, consisting mainly of the blue-gray magnetite, Fe3O4. Meanwhile, rust is formed by the reaction of iron/steel and oxygen in the presence of water or air moisture.
Even though we are talking about two different terms, welding through mill scale, while possible, can present several potential issues and challenges similar to welding rusty metals. The scale behaves as a thermal and electrical insulator. This behavior restricts puddle fluidity and creates arc instability.
Additionally, the scale has impurities like oxygen that it can release into the steel. Oxygen ions are released into the weld metal as the mill scale degenerates. The oxygen will increase the porosity, which weakens the joint. Mill scale often creates an uneven and textured surface. Welding on such surfaces can lead to inconsistent weld bead profiles and difficulty achieving a smooth, uniform finish.
How To Remove Mill Scale Before Welding?
Removing the mill scale before welding is crucial to ensure a clean, smooth surface for solid and reliable welds. Like rust, mill scale can be light and present only on the surface. However, fresh mill scaling is well-adhered to the surface, so you will have trouble removing it. Therefore, to remove the mill scale before welding, you can employ the following methods:
- Grinding: An angle grinder is always the best way to remove most contaminants, including the mill scale, from the surface. With the light scale, you can use nylon brushes and/or wheels or stripping discs. However, if the layer is more persistent, a silicon carbide disc or a wire wheel will do a better job.
- Blasting: Like rust, blasting will do an excellent job cleaning, polishing the surfaces, and removing the persistent layers of mill scale. This is a quick and cost-efficient method for mill scale removal, but it requires specialized equipment that is not readily available.
- Flame cleaning: Due to the nature of the formation of the mill scale, you can use the very same temperature to work against it and remove it. A hot oxyacetylene flame will remove the mill scale, but it can also help with other contaminants. However, the applications are limited to thicker pieces that cannot warp due to high heat.
- Chemicals: if you can handle them correctly, chemicals are also an excellent way to remove mill scale. Acid-like chemicals react with mill scale on a molecular level and remove it, but keep in mind that the reaction can carry on and damage the base metal and harm you or your environment.
Conclusion
Cleaning the metal before welding is a fundamental step that must be considered in the welding process. Whether dealing with rust, mill scale, paint, or other contaminants, the quality and integrity of a weld largely depend on the cleanliness of the base metal.
By diligently following the appropriate methods for surface preparation, you can be sure that you are working on a clean canvas, ready to create strong, reliable, and safe welds. While the process may require additional time and effort, the benefits in terms of weld quality, longevity, and safety far outweigh the initial investment.
🧐How To Clean Metal Before Welding - FAQ
1. How to remove galvanized paint or coating?
- Brushing: If you are dealing with loose or flaking galvanized paint or coating, you will only need a wire brush. If the galvanized paint is more persistent, you can use power tools with a wire brush attachment. Brushes are also recommended in tight areas where you cannot use power tools or grinders.
- Grinding: Grinding is almost always a better solution for thicker or persistent galvanizing. An angle grinder equipped with an aluminum oxide or silicon carbide abrasive wheel will remove the galvanized layer thoroughly, and you should do it all around the weld joint. However, grinding can be potentially hazardous since zinc dust or particles can fly around the workshop. Also, the abrasives can heat the zinc, producing limited fumes. Additionally, if you grind too deep, you can incorporate zinc into the joint, which can compromise the integrity of the weld. Nonetheless, grinding is a go-to method for zinc-layer removal if you do it right.
2. How to remove paint before welding?
- Brushing: As noted, most of the paint can be easily removed with a wire brush. Brushing is a relatively easy and quick operation that will remove the paint from the surface quickly so you can return to welding in no time.
- Sanding: Sanding is another fairly easy and quick procedure that will help you remove the paint from the metal before welding. However, hand sanding can be time-consuming and hard, especially on larger pieces, so most welders use a sanding disc or wire wheel mounted on an angle grinder or other power tools.
- Paint Strippers: Paint strippers are chemicals that can penetrate through multiple layers of paint and weaken the adhesion between it and the surface. You apply them to the paint, let them react, and easily strip the paint with a brush. These can be used on various types of paint and are recommended for thicker and persistent paint layers.
3. How do you remove rust before welding?
- Removing light, surface rust before welding: rust-colored deposits on the surface of the metal present the first stages of rust or surface rust, which can typically be easily removed. Like with paint, you can use a wire brush or sanding paper or discs to remove it. Naturally, it comes off easily, but removing surface rust can create hazardous rust dust, so make sure you wear a mask.
- Removing heavy, thick rust before welding: in further stages of development, rust slowly penetrates the surface of the base metal and creates thicker layers. On the top of the surface, the rust scale is formed, but that's just the tip of the iceberg. While you can remove the scale with sandpaper or sanding wheels and brushes, the thicker layers require grinding, blasting, or chemicals. Angle grinders with a flap disc or a sanding disc will quickly remove large-scale heavy rust but be aware of the dust and other particles. Acid-like chemicals are also a good solution, but you should handle them carefully. Finally, abrasive blasting or sandblasting is used to clean highly rusty pieces, but its equipment is often reserved for industrial applications.
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