Many metal fabrication shops will tell you to avoid designing overlapping weld joints. This is where one piece of metal, usually sheet steel, is laid on top of another and welded together. The problem with this type of joint, (which shouldn’t be confused with the overlap weld defect), is that it’s very susceptible to rusting. However, if overlapping joints are essential for meeting the design objectives of your fabrication, there are ways to prevent rust. This blog explains how to go about producing durable overlapping joints. If you get to the end and feel you need more information, please contact us to discuss your specific project or requirements.
What is an Overlapping Weld Joint?
Welding instructors teach five primary types of welded joints: butt, tee, corner, lap, and edge. Where then does the overlapping joint come in?
The answer is that an overlapping weld joint is where one piece of metal covers up another. A lap joint is a type of overlapping joint, as is an edge joint. A third type of overlapping weld is a less common joint called the plug weld. Like a lap joint, this is where one piece of metal is laid over another, but in this case,e the uppermost piece has a hole in it. A weld is made in this hole to join the upper and lower pieces.
Lap welds and plug welds are made to join otherwise separate pieces of sheet metal. They’re used in car restoration and bodywork repair. Resistance welding requires overlapped sheet metal.
In the metal fabrication industry, we try to avoid them, because they tend to rust. On occasion though, usually when the designer wants an imperceptible joint between two sheets, they’re unavoidable.
Edge joints can be a little harder to avoid. These can be required in sheet metal, and also in angle. An edge joint is somewhat like a seam where each of the pieces being welded is bent through 90° and the two faces butted up together.
Why Overlapping Weld Joints Rust
Iron likes to bond with oxygen, and when this happens, the resulting iron oxide occupies more volume than iron alone. The result is ugly orange streaks and flakes as the steel decays.
Rust is prevented by covering the steel to protect it from oxygen, but coated steel can’t be welded. So, overlapping joints always involve some bare, uncoated metal.
Powder coating or painting the fabrication after welding encloses the overlapping region, but there are two problems. First, the viscosity of these coatings so too high for them to get drawn into the region between the pieces. And second, there will be air in there, and that air will contain moisture. It will take time, but eventually, orange streaks will appear from the weld. Plus, corrosion may be accelerated by the remnants of the cleaning and flux solutions applied to make the weld.
Galvanized steel is not immune from this problem either. Galvanizing is the process of applying a layer of sacrificial zinc to the surface of the steel. This is a very effective means of preventing corrosion until two galvanized pieces are welded. Zinc melts at 787°F and boils or vaporizes at 1,665°F. Low-carbon steel, however, doesn’t melt until 2,570°F. So when welding galvanized steel, the zinc is long gone before the weld is made.
To conclude this section, note that galvanizing after welding doesn’t solve the rust problem either. As with paint and powder, liquid zinc is too viscous to penetrate the overlapping region.
Preventing Rust in Overlapping Weld Joints
Having explained the problem, let’s look at four ways to prevent it.
1. Use seal welds
The overlapping joint certainly has two sides, and possibly four. The idea of a seal weld is to seal up the overlapping area. This means putting a weld bead along the edge opposite the main weld, and along any other open edges to completely enclose the overlapping region.
This means more welding time and filler, and it can also affect how the fabrication responds to loads and temperature changes. Making an additional one to three seam welds increases stiffness, and may result in loads being transmitted differently through the structure. It will also alter how it expands and contracts and can lead to unexpected deformation.
2. Use caulk
Caulk isn’t just for sealing gaps in house siding: it can do the same job in an overlapping weld joint. As with making seam welds, this carries a cost and can get messy, but it won’t affect the structural integrity or cause distortion. However, caulk won’t last as long as the weld, so at some point, a replacement will be needed. In addition, silicone-based caulks are incompatible with many paints and polymers, so coating will be a challenge.
3. Use weld-through primer
This technique is widely used in the automotive repair industry. Weld-through primer is a zinc-based paint that’s applied across the overlap area. The zinc melts as the weld is made, coating both surfaces with corrosion-resistant metal.
This sounds good in principle, but there is a potential problem: there’s no non-destructive way to inspect the overlapped areas for zinc coverage. This may mean making a test weld with a measured application of primer, then sectioning the joint to check for coverage, and modifying the primer application as necessary.
4. Find alternative types of welded joint
We titled this blog, “Four Ways of Preventing Rust in an Overlapping Weld Joint”, so this might seem like a cheat, but honestly, the only really dependable way to prevent rust is to not overlap metal. Design your fabrication with butt welds instead. You might notice that storage tanks are usually fabricated with butt welds when overlapping joints might seem easier to make, and this is the reason.
Discuss Your Fabrication Needs With Us
When you’re designing a fabricated structure there can be occasions where an overlap seems the most logical type of weld to specify. As we’ve explained here though, such a joint is highly likely to start rusting, perhaps after a relatively short time in service.
There are ways to reduce the risk of rust and to postpone its occurrence, but they will add to the cost of your project. That’s why our advice is to find a different way to satisfy your design objectives. It may not add as much cost as you think.
We’ve been in the metal fabrication industry a long time, and as a family-owned business blessed with a stable workforce, there’s a lot of experience under our roof. If you’re working on a fabrication and could use some advice, contact us before your design gets frozen or locked in: we could save you time, money, and the annoyance of ugly orange streaks spoiling your structure.