High-Temperature Hydrogen Attack (HTHA) is one of the most critical damage mechanisms governed by the API 571 standard. It affects carbon and low-alloy steels exposed to high-pressure hydrogen at temperatures above 200°C.
Mechanism of HTHA
HTHA occurs when atomic hydrogen diffuses into the steel and reacts with carbon (carbides) to form methane gas:
Chemical Equation / FormulaFe_3C + 4H → 3Fe + CH_4
Since the methane molecule is too large to diffuse out of the steel, it accumulates at grain boundaries, building up extreme internal pressure. This leads to:
- Decarburization: Loss of steel strength due to carbide breakdown.
- Micro-fissuring: Formation of microscopic cracks along grain boundaries.
- Fissuring and Cracking: Coalescence of fissures into macro-cracks, leading to sudden, catastrophic failure.
Nelson Curves and Operating Limits
The operating limits of steel in hydrogen service are guided by the Nelson Curves (API RP 941). Following several industry failures, major updates have shifted the safe operating limits for carbon steels down, placing many existing assets under closer inspection requirements.
Advanced NDT Detection Techniques
Standard NDT techniques (like radiographic testing or basic ultrasonic testing) cannot detect early-stage HTHA. Modern inspection campaigns utilize a combination of advanced ultrasonic methods:
- Time of Flight Diffraction (TOFD): Extremely sensitive to spatial micro-fissures and cracking.
- Phased Array Ultrasonic Testing (PAUT): Used to scan weld zones and heat-affected zones.
- Advanced Ultrasonic Backscatter Technique (AUBT): Specifically designed to measure backscatter and velocity ratios to detect early-stage HTHA before visible cracking occurs.