An arc strike is an unintentional mark left on a metal surface when a welding electrode or torch briefly contacts the base metal outside the intended weld zone. It creates a small, localized spot where the metal melted and rapidly re-cooled, leaving behind a hard, brittle area that can become a starting point for cracks. In structural and pressure vessel work, arc strikes are considered serious defects that often require repair.
How Arc Strikes Happen
Arc strikes occur when the welder accidentally touches the electrode to the workpiece in the wrong spot. This can happen in several ways: striking the arc to start welding but missing the joint, dragging a live electrode across the surface while repositioning, or letting the electrode contact nearby metal when working in tight spaces. With stick welding (SMAW), some welders intentionally strike their electrode on the base metal near the joint to initiate the arc, then drag it into position. This common shortcut leaves arc strikes every time.
Loose or damaged ground clamps can also cause arc strikes. If the grounding connection is poor, the electrical current may arc at the clamp contact point, creating the same type of localized damage on the metal surface. TIG and MIG welding processes produce arc strikes less frequently because their arc-starting methods are more controlled, but they still occur when the welder loses focus or works in awkward positions.
Why Arc Strikes Are Dangerous
The real problem with an arc strike isn’t the visible mark on the surface. It’s what happens to the metal underneath. When the arc contacts bare metal for even a fraction of a second, it heats a tiny area to its melting point. That spot then cools almost instantly because the surrounding cold metal acts as a massive heat sink. This rapid heating and cooling cycle, called a quench, changes the metal’s microstructure in that small zone.
In carbon steel and alloy steels, this quench effect produces an extremely hard, brittle layer known as untempered martensite. The harder and more brittle this layer becomes, the more susceptible it is to cracking. High-strength steels and thicker materials are especially vulnerable because their chemistry makes them more prone to forming these hard zones. An arc strike on a high-carbon or low-alloy steel pipe carrying high-pressure fluid can develop a crack that propagates through the wall thickness over time, particularly under cyclic loading or in corrosive environments.
Arc strikes also introduce hydrogen into the affected area if the electrode was a type that carries moisture. Hydrogen trapped in that hard, brittle zone can cause delayed cracking, sometimes called cold cracking, which may not appear for hours or even days after the arc strike occurred. The combination of a hard microstructure, residual stress, and trapped hydrogen is one of the most reliable recipes for cracking in steel.
What an Arc Strike Looks Like
Visually, an arc strike appears as a small, rough, discolored spot on the metal surface. It typically looks like a tiny crater or a spatter mark, sometimes with a bluish or darkened halo from the heat. On polished or coated surfaces, the damage is obvious. On rougher mill-scale surfaces, small arc strikes can be easy to miss without close inspection.
Under the surface, the heat-affected zone from an arc strike is shallow but distinct. If you were to cross-section an arc strike and examine it under a microscope, you’d see a thin layer of re-solidified metal on top, a narrow band of transformed microstructure beneath it, and then a transition back to the unaffected base metal. Micro-cracks within this zone are common, even when no cracks are visible to the naked eye.
Code Requirements and Inspection
Most welding codes and standards treat arc strikes as rejectable defects that must be addressed. The AWS D1.1 Structural Welding Code, ASME Boiler and Pressure Vessel Code, and API pipeline standards all include specific language about arc strikes. In pipeline work governed by API 1104, arc strikes outside the weld zone are explicitly prohibited.
The typical repair procedure involves grinding out the arc strike to remove all affected material, then inspecting the ground area to confirm no cracks remain. Magnetic particle testing (MT) or dye penetrant testing (PT) is commonly used to verify the repair. If the grinding reduces the wall thickness below the minimum allowed, the area may need to be welded and re-inspected. For critical applications like nuclear or high-pressure piping, the repair process can be surprisingly involved for such a small defect.
Inspectors specifically look for arc strikes during visual examination of welds. They check not just the weld itself but the surrounding base metal, ground clamp locations, and any area where a welder might have accidentally contacted the surface. On large fabrication projects, repeated arc strikes from the same welder can raise questions about workmanship quality overall.
How to Prevent Arc Strikes
The most straightforward prevention method is to always start the arc within the weld joint itself, or on a run-off tab that will be removed later. Many welding procedures specify that the arc should be initiated only in the weld groove, on tack welds, or on previously deposited weld metal.
- Use strike plates: A small piece of scrap metal placed near the joint gives you a safe surface to initiate the arc before moving into the weld zone.
- Check your ground clamp: Make sure the clamp has a clean, tight connection to the workpiece. A loose clamp arcing against the surface creates the same damage as an electrode strike.
- Cap your electrode: When moving between joints with a stick electrode, keep the electrode away from the workpiece. Some welders place a small insulating cap over the tip.
- Use proper arc-starting features: TIG welders with high-frequency start or lift-arc capability reduce the chance of accidental strikes. MIG welders with run-in speed control help prevent the wire from jabbing into the base metal at the start.
In shop environments, wrapping or taping areas adjacent to the weld zone with protective material provides a physical barrier against accidental contact. This is common practice in aerospace and nuclear fabrication where the consequences of an arc strike on the finished component are severe enough to scrap the part entirely.
Arc Strikes on Different Materials
The severity of an arc strike depends heavily on what metal you’re working with. On mild steel with low carbon content, the hardened zone from an arc strike is relatively modest and less likely to crack on its own. The risk increases significantly with higher-carbon steels, chrome-moly alloys, and quenched-and-tempered steels, where the rapid cooling produces much harder and more brittle microstructures.
Stainless steels present a different concern. An arc strike on austenitic stainless steel (the most common type, like 304 or 316) can sensitize the affected area, making it vulnerable to a specific type of corrosion called intergranular attack. In chemical processing or marine environments, that tiny sensitized spot can become a corrosion initiation site long before the rest of the metal shows any degradation.
On aluminum, arc strikes are less structurally dangerous but can still cause cosmetic defects and local contamination. For titanium and other reactive metals, an arc strike in open air causes immediate surface contamination from atmospheric gases, producing a hard, brittle spot that compromises the metal’s corrosion resistance and fatigue life. Titanium welding is typically done under full inert gas shielding for exactly this reason, and any arc strike usually means scrapping or extensively reworking the affected area.