Welding rusted metal can be challenging but is very doable with the right techniques. The main steps are: 1) Clean the metal with a wire brush or sandpaper to remove loose rust and paint 2) Use a rust converter or phosphoric acid to convert remaining rust into an inert layer 3) Weld in short segments to prevent heat damage and warping 4) Use higher amperage settings to penetrate the rust layer 5) Consider preheating thick, heavily rusted sections 6) Use an abrasive flap wheel to remove slag and remaining rust between passes 7) Apply a rust-inhibiting primer after welding to prevent recurrence of rust
Rust poses a significant challenge when welding metal. Not only does it weaken the base metal, but it also interferes with arc starts, reduces weld penetration, and leads to porous, contaminated welds if not properly addressed. However, with the right preparation methods and adjustments to your welding technique, you can tackle rusted metal with great results.
This comprehensive guide will walk through a step-by-step process for welding rusted metal. With some added awareness and a few extra steps, you can salvage rusted items and prevent recurring rust problems in your welds. Let’s get started!
Step 1: Clean the Metal Surface
The first critical step is to clean the metal to remove all loose rust, scale, paint or other coatings from the area to be welded. This will:
- Prevent porosity problems from contaminants burning into the weld
- Improve weld penetration by exposing clean metal
- Allow paint/primer to adhere after welding
Use the following cleaning methods depending on the severity of rust:
For light surface rust:
- A wire brush attachment on an angle grinder or drill is ideal for removing loose rust and paint. Wear eye protection, gloves, and a dust mask.
For heavier rust buildup:
- Sandblasting offers the quickest way to remove heavy rust scale and old paint/coatings
- For hand cleaning, use an assortment of coarse flap wheels, grinding wheels, or sandpaper. 80-120 grit works well.
Get into corners and crevices:
- A die grinder with a rotary file bit or small wire wheel can clean inside corners and other hard to reach areas
Pro Tip: When possible, clean at least 1 inch around the area to be welded to prevent arc blow effects from remaining rust at the weld zone.
Step 2: Convert Remaining Rust
Once you’ve removed all the loose rust and exposed clean metal, there will inevitably still be some remaining rust embedded in pits or pores of the surface.
While it’s impossible to remove every spec of rust, you can convert the remaining rust into a protective chemical layer called magnetite (black iron oxide) using a rust converter.
Popular options include:
- Loctite Naval Jelly
- Corroseal Rust Converter
- Permatex Rust Converter
These converters contain tannic, phosphoric, or oxalic acid which reacts with the rust to form an inert black layer. This prevents further rust formation but also forms a primer-like coating to weld over.
Apply a thin coat per the product instructions and allow to fully cure before welding. The metal may now appear black – this is the converted magnetite layer.
An alternative is to use straight phosphoric acid as a rust pretreatment. Phosphoric acid has a similar rust converting reaction. Wipe or brush on a diluted 10-30% phosphoric acid solution and let it sit for about 5 minutes before welding.
The benefit of a rust converter is convenience, while phosphoric acid is extremely economical if you already have it on hand. Both will improve your welding results on rusty metal.
Step 3: Weld in Short Segments
Once prepped, it’s time to start laying down welds. But welding long continuous beads on rusty metal can lead to excessive heat buildup, warping, and burn through.
Instead, make short stitch welds 2-4 inches long and allow time to cool between passes. This localized heating minimizes distortion. Leapfrog to the next segment and work your way along the weld joint.
For thicker metal or full penetration corner/tee joints, you may need to make two or three passes over each segment before moving to the next. Either way, pausing regularly prevents overheating and gives any remaining rust time to burn out of each section.
Editor Note: If able, position the work so gravity or clamps pull the metal together as it tries to warp or distort from welding. This counteracts shrinkage forces.
Step 4: Increase Your Amperage
Rusty metal requires more welding current to push through the rust layer, overcome surface contamination, and achieve good fusion.
As a rule of thumb, increase your amperage setting by 20-40% over normal settings. Higher amperage increases weld penetration and heat input to burn through rust.
Don’t be afraid to turn up the heat! Just monitor your puddle and adjust until you achieve nice wetting action and visible fusion into the base metal.
Of course, each situation is unique. You may need to go higher on heavy sections with thick rust. Or if you preheated the workpiece first (see next step), you won’t need as much amp boost. Dial it in based on the results you see.
Step 5: Consider Preheating
For thicker metal – 3/8 inch and up – or heavy, pitted rust, preheating before welding can help in several ways:
- Slows the cooling rate – Preheating reduces the abrupt temperature difference between the weld zone and base metal for less cracking risk.
- Lowers hydrogen absorption – Hotter metal accepts less hydrogen as it cools, preventing hydrogen assisted cold cracking.
- Improves weld penetration – Preheating reduces the amount of arc heat needed to penetrate the thick metal.
- Burns off mill scale or rust – Preheating from 300-500°F burns away impurities for a cleaner weld.
Use an oxy-fuel torch or induction heater to preheat up to several inches around the weld area. Measure preheat and inter pass temperature with temperature crayons, thermometer guns, or thermocouples.
Pro Tip: For heavy rust removal, some welders temporarily heat the metal to red hot (1300°F) to completely burn off all rust before welding. Use caution around flammable areas!
Step 6: Remove Slag and Rust Between Passes
As you make multiple weld passes, slag will build up along with remnants of rust or mill scale. Stop to remove all slag and debris before applying the next bead.
Use a stainless steel brush or abrasive flap wheel to scrub away residue between passes. This prevents inclusions and contamination in your welds. Debris can hinder fusion or be trapped in the weld, leading to porosity and cracking.
Your end weld profile will also look much cleaner if you take the time to brush thoroughly between each new pass. Don’t underestimate this step!
Step 7: Apply Rust-Inhibiting Primer
Once welding is complete, take measures to prevent recurrence of rust. Even with the best prep, remnants of rust will likely remain in the pits or pores of the metal.
Applying weld-through rust-inhibiting primer provides an extra layer of protection. The zinc or iron phosphate compounds in the primer react with metal to stop rust.
Great options include:
- Rustoleum Rusty Metal Primer
- POR 15 Rust Preventive Coating
- Eastwood Internal Frame Coating
Use self-etching primer designed for use without sanding bare metal. Follow application and safety instructions carefully.
Curing the part at 250°F accelerates the protective reaction. But even air drying primer will help achieve long-lasting rust prevention after welding.
With some added preparation and adjustments, welding over rust can become standard practice for your shop. Simply stay vigilant in your technique, and inspect welds closely for signs of contamination or lack of fusion. I hope you found these steps helpful for salvaging rusty metal and preventing recurring rust problems. Let me know if you have any other tips for successful rust welding! 👩🏭