What Is Arc Strike? Surface Damage
An arc strike is a surface discontinuity caused by the accidental or intentional contact of a welding electrode, welding arc, or ground clamp with the base metal outside the weld zone. The momentary arc melts a small area of the surface and then quenches it rapidly, creating a localized hardened zone that may contain micro-cracks. Arc strikes are treated as serious defects in pressure piping and sour service applications.
| Aspect | Details |
|---|---|
| Definition | Localized surface damage from unintended arc contact outside the weld joint |
| Appearance | Small crater or discolored spot, typically 2-10 mm diameter |
| Metallurgical effect | Rapid heating and quenching creates a hard, brittle martensitic zone |
| Hardness | Can exceed 400 HV in carbon steel (well above the 248 HV NACE limit) |
| Crack risk | Micro-cracks may form in the hardened zone, acting as fatigue initiation sites |
| Corrosion risk | Hard zone is susceptible to sulfide stress cracking in sour (H2S) service |
Key Code Requirements
| Code/Standard | Requirement |
|---|---|
| ASME B31.3 | Arc strikes must be ground smooth and examined by MT or PT |
| ASME Section VIII | Requires removal and NDE of affected area |
| AWS D1.1 | Arc strikes outside the weld must be ground smooth |
| API 1104 | Arc burns on pipe surface must be removed |
| NACE MR0175/ISO 15156 | No arc strikes permitted; area must be ground and hardness tested |
| EN 1090-2 | Arc strikes not permitted on structural steelwork above EXC1 |
| Client specs (typical) | Ground flush, MT/PT inspected, hardness-tested in sour service |
Why Arc Strikes Are Dangerous
The danger of an arc strike is not the visible crater but the metallurgical damage underneath. The rapid thermal cycle creates an untempered martensite microstructure in carbon and alloy steels that is both hard and brittle. In sour service (H2S environments), this hard zone is highly susceptible to sulfide stress cracking (SSC). Even in non-sour service, the micro-cracks in the hard zone can propagate under cyclic loading or thermal fatigue.
Prevention
- Use electrode holders with proper insulation to prevent accidental contact
- Attach the ground clamp directly to the workpiece, not through the pipe support or nearby structure
- Train welders to strike the arc only within the weld joint preparation
- Use scratch-start or lift-arc TIG ignition instead of touch-start methods outside the joint
- On stainless steel and nickel alloy piping, arc strikes cause localized sensitization and pitting corrosion, requiring even more stringent removal
All arc strikes must be documented and addressed per the project inspection and testing plan.
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