Welding Corten (Weathering Steel): Filler Choice for Bare vs Painted Service

Weathering steel welds with the same processes and roughly the same procedure as A36. The decision that trips people up is filler metal, and it comes down to one question: will the finished weld sit bare and exposed, or will it be painted, coated, or hidden. For single-pass welds, painted work, and most non-exposed joints, plain ER70S-6 MIG wire or E7018 stick makes a sound, full-strength joint. For bare exposed multi-pass welds where the weld has to weather and patina like the parent steel, you need a low-alloy filler carrying the W designator.

Weathering steel is the generic name for the family covered by ASTM A588, A242, and A606 (A847 covers tubing, A709 Grade 50W the bridge version). Corten is a brand name, originally US Steel, that became the household word for it the way Bondo did for body filler. The steel is mild steel alloyed with small amounts of copper, chromium, nickel, and phosphorus. Those alloys cause it to form a tight, adherent rust layer (the patina) that seals the surface and slows further corrosion, which is why architects and sculptors use it bare with no paint. Mechanically it is close to A36 and A572 Gr 50, with a minimum yield around 50 ksi for the common A588 plate.

Is Weathering Steel Hard to Weld?

No. The carbon equivalent on A588 and its cousins runs in the low 0.40s, similar to A36, so it sits in the readily weldable range without exotic procedures. You can run MIG, TIG, stick, or flux-core on it. The weldability question almost never the problem. The problem is matching the weld to the base metal for corrosion and appearance once the part goes outside and starts to rust.

Two things are at stake on a bare exposed weld:

• Corrosion rate. Plain carbon-steel filler has none of the copper, chromium, and nickel that make weathering steel self-protecting. A weld made with straight ER70S-6 will rust faster than the steel around it. On a thick exposed multi-pass weld that difference compounds over decades.
• Color. Every weld eventually rusts to some brown, but a plain-carbon weld patinas to a different shade and at a different rate than the base metal, leaving a visible stripe. On an architectural panel or a sculpture that stripe is the whole problem.

Both of those issues come from dilution. In a single thin pass, a lot of the surrounding weathering steel melts into the puddle and carries its alloying elements with it, so even a plain-carbon weld ends up partly alloyed and weathers close to the parent. In a heavy multi-pass weld, the later passes are made almost entirely of filler metal sitting on top of earlier filler, so very little base metal dilutes in. That is exactly the case where you need the alloy to come from the filler instead.

Filler Metal for Weathering Steel by Service

The honest rule is service-based, not one-size-fits-all.

The W-series fillers (the designator literally stands for weathering) put copper, chromium, and nickel into the deposit so the weld metal itself forms a patina close to the base steel. E8018-W2 is the common stick electrode. For wire feed, manufacturers sell weathering-specific low-alloy MIG solid wires and gas-shielded flux-core wires under names like Cor-Match 8018-W and the 80-CW and 810-W series, classified to AWS A5.28 (gas metal arc) and A5.29 (flux-cored) low-alloy specs. They run nearly identically to their plain-carbon equivalents on the machine. You set them the same way you would set ER70S-6 or E71T-1 of the same diameter.

The nickel-bearing fillers like ER80S-Ni1 are a middle option. About one percent nickel buys meaningful corrosion resistance for less than a full W-series wire, and on a structural joint that sees normal dilution it is often enough. This matches the note many wire data sheets carry, that weathering applications may call for a nickel-bearing wire to meet the higher mechanical and corrosion properties. Whether Ni1 is sufficient or a full W filler is required on a given exposed structural weld is a code-and-engineer question, covered next.

What AWS D1.1 Requires for Exposed Bare Welds

For structural weathering-steel work, the filler choice is not yours to make alone. AWS D1.1 Structural Welding Code - Steel contains a dedicated table of filler metal requirements for exposed bare applications of weathering steels. It lists which filler classifications are acceptable, and for some processes it limits how plain-carbon filler can be used (single-pass only, for instance) versus where a weathering-grade filler is mandatory. The table has been renumbered between editions, so look it up in the edition your job is written to rather than trusting a number from a forum post.

The takeaway for the shop: if the steel is structural, exposed, and bare, the governing code and the engineer of record (EOR) decide the filler, the procedure, and the acceptance criteria. Do not substitute a cheaper filler to save a few dollars on a spool. Match the welding procedure specification (WPS) you were handed, and if there is no WPS for the weathering-steel exposed condition, that is a question for the EOR, not a guess at the bench. Patina and color matching are described here as typical behavior of the system, not a guaranteed appearance. Real weld color depends on the steel lot, the environment, sheltering, and how the part drains.

Procedure: Same as Mild Steel, With Two Habits Added

Everything you do welding A36 structural steel carries straight over. The same preheat logic, the same joint prep, the same settings for a given thickness. The phosphorus that helps the patina form does make the weld metal a little more sensitive to hot cracking and the heat-affected zone a little more sensitive to hydrogen, so two habits are worth keeping.

Lean toward low-hydrogen and preheat. Use low-hydrogen consumables (E7018 or E8018-W with proper rod-oven storage, or solid wire) and keep the preheat at least at the A36 numbers for the thickness, not below them. The W-series fillers respond well to the same preheat practice you would use on A36 of equal section. Verify the requirement against AWS D1.1 and your WPS, since restraint and thickness change the answer.

Clean it like you mean it. Weathering steel arrives with mill scale and often a head start on surface rust. Grind or blast the weld zone back about an inch on both sides the same way you would for code work on A36. Detail on the practical filler tradeoffs lives in the mild steel filler metal selection guide, and the choices there apply directly once you have decided whether your weathering-steel joint needs a W filler or not.

One thing that is specific to weathering steel: design the joint so it drains. The patina only stays protective if the surface wets and dries in cycles. Crevices, lap joints that trap water, and pockets that stay damp will keep corroding instead of sealing, and on weathering steel a wet pocket can stain everything below it with rust runoff. Detail lap and corner joints to shed water, seal-weld where water would otherwise sit, and avoid horizontal ledges that hold debris.

Settings for Common Weathering-Steel Work

Because the base metal behaves like A36 and the W-series fillers run like their plain-carbon counterparts, you can start from standard mild-steel numbers. These are general starting points to dial in against your machine, your wire, and the WPS, not final values.

Run the W-series and Ni1 wires drier and a touch hotter than you might expect only if the maker’s data sheet says so. Otherwise the parameters that work for ER70S-6 of the same diameter work here. The slag on E8018-W behaves like 7018, peeling in strips on a clean flat weld.

Common Problems Welding Weathering Steel

The weld is a bright stripe and stays that way. Expected at first. A fresh weld on weathering steel is glossy gray against the matte brown base, and the contrast is sharp for the first months. With a W-series filler the stripe fades as the deposit patinas in over a couple of years. With plain-carbon filler on a bare exposed multi-pass weld the stripe does not fully close and the weld keeps corroding faster. If color match matters, that is the cost of having used the wrong filler.

Rust streaking below welds and joints. Almost always a drainage or crevice problem, not a welding problem. Water sitting in a lap or pocket runs rust down the face. Fix it in the design: seal-weld trapped seams, add drain paths, avoid water-holding ledges.

Porosity. Same causes as on A36. Check gas coverage and flow (20-25 CFH for 75/25), kill drafts, and clean the mill scale and any surface rust off the joint. Weathering steel that has been outside often has more surface rust than fresh A36, so cleaning matters more.

HAZ or weld cracking on thick restrained joints. The phosphorus and the low-alloy filler nudge the susceptibility up. Preheat per the code for the thickness, use low-hydrogen filler with proper storage, and do not weld restrained thick sections cold. Any crack in a structural or pressure weld is a rejectable defect and needs inspection and engineering disposition, not a cosmetic grind-and-fill.

When You Are Building Non-Structural

If you are fabricating a planter, a privacy panel, a sculpture, or a fire screen out of weathering steel for your own yard, the code does not apply and your filler choice is just a corrosion-and-looks tradeoff. Plain ER70S-6 on a thin single-pass weld will weather close enough that most people never notice. If you want the welds to vanish into the patina over the years on a bare exposed piece with visible multi-pass beads, spend the extra on a W-series wire or E8018-W rod. For anything that is part of a building, a bridge, or a load path, the AWS D1.1 structural welding code and the engineer of record set the filler and the procedure, and the mill certs for the steel tell you which grade you actually have. This page is general fabrication information, not a substitute for the governing code, the manufacturer’s filler data, or the EOR’s WPS.