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One of the golden rules to creating a strong weld is to have a clean work surface. However, you may not always be lucky enough to have a workpiece that’s pristine. The problem is that metals possess great strength, but they also rust once they contact water or moisture. Thus, one of the most common problems you’ll face as a welder is dealing with rusty materials.
Should you weld on rust or rusty material? What will be the weld quality? Will the weld fail to hold the rusted metal pieces together? These are the sorts of questions you may be asking yourself when working with a rusty workpiece. While welding on rust is certainly possible, you should consider a few important factors to ensure a high-quality weld. Let’s dig deeper to understand how to deal with such welding projects efficiently.
Welding on Rusty Material is Not Easy
Most welders prefer to stay away from rusted materials because of the inherent risk of material failure. However, if you’re dealing with rusty metal, here are a few things you should be aware of.
The Weld May Not Be Aesthetically Pleasing
Welding rusty material often leads to unsightly welds. Although they’re by no means weak, these welds don’t possess the clean lines that you’re probably looking for. Luckily, in these cases, a good-looking weld is not usually the highest the priority, so long as the job gets done.
Rust Can Severely Affect Weld Quality
Welding on rusty materials is risky because the rust can directly affect the weld quality. During welding, the rust particles can mix with the weld pool and lower its fluidity, thereby preventing the metals from mixing well. This issue can cause weaker welds that may fail prematurely when under load.
Thin Metal Can Go beyond the Point of Repair
For a weld to work, the arc must be able to contact the bare metal. However, deep rust can easily run all the way through thin metal sheets, completely disintegrating the base metal. In such cases, welding is neither possible nor advisable.
What is the Right Type of Weld Method for Welding Rusty Metals?
When it comes to welding rusty metals, the flux-cored welding method usually produces the best results. The formation of the flux will help you clear the debris while the deep penetration ensures that the arc can catch the bare metal. Therefore, your first choice should be stick welding paired with a 6010 or 6011 rod.
However, if it’s possible for you to remove the rust from the metal surface, you can also use MIG welding to produce a much better weld. For most applications, a wire diameter of 0.030 inches does the job, while for thinner metals, the ideal diameter is 0.023 inches. For thicker metals, you can choose diameters of 0.035 to 0.045 inches.
To remove the rust from the workpiece, use a wire brush or grind the surface to remove the top layer, taking care to prevent any further weakening of the structure.
Welding Rusted Metal: The Process
By now, you already know that welding on a rusted material is not as straightforward as welding on bare metal. There are a few things you need to consider to achieve a good quality weld. Here are some of the important steps you should follow.
Step 1: Check the Workpiece Thoroughly and Remove Rust if You Can
The first step is to check the workpiece to understand the extent to which it has rusted. Rust formation happens in stages, and the metal will have different outer surface traits based on these stages.
For Stage 1 rust, you’ll notice reddish-brown spots on the metal. These spots are not uniform but somewhat scattered all over the metal surface. This type of rust is primarily superficial, and you can easily remove it.
Stage 2 rust is no longer superficial. The rust will start degrading the metal, and scaling begins. Depending on the metal type, the etching and pitting will either be irregular or uniform. If your workpiece shows signs of second-stage rust, you can remove the scaling using a wire brush or by gently grinding the top layer from the metal.
In Stage 3, a large portion of the metal undergoes scaling. The structural integrity of the metal object will be in question. Based on the thickness of the metal, the rust may weaken it partially or entirely. At this stage, removing the scales for welding should be performed with caution. Aggressive removal of the rust may leave holes in the metal structure.
As a final tip, it’s worth noting that spraying primer both before and after the weld can help prevent corrosion.
Step 2: Choose the Correct Welding Method
The welding method depends on the condition of the workpiece. If rust removal is possible, you can MIG weld the metal. Otherwise, use stick welding for projects where you cannot grind the rust off. Stick welding allows you to weld even when there are impurities present on the metal surface, and you can clean out those impurities later from the welding flux. See the section above on types of welding method for more details.
Step 4: Check the Weld Quality
One of the major issues with welding on a rusty metal is that impurities from the rust can affect the weld quality. Porosity is the first thing that you should look for. It manifests as little pinholes on the weld’s surface. This sign indicates that gas bubbles are trapped within the weld, which ultimately reduces the weld strength. If you see these pinholes, grind the weld down and repeat the welding process.
Step 5: Make Additional Passes as Necessary
It’s always good to make additional passes to achieve the desired weld quality. Additionally, with each new pass, make sure to clean the weld and inspect it for any defects.
Welding on rusted metal can be cumbersome, and the chances of a successful weld depend on several factors. However, with the right approach, it is possible to successfully complete such welding projects.
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