Zhuojin Pipe Fitting Co., LtdOxide Film Characteristics on Low Alloy Steel and Stainless Steel Overlay Welds in High Temperature Water with Different Oxygen Content
Literature Overview
This paper by Xiong Qi and colleagues from Shanghai University, published in the journal Corrosion and Protection (2018, Vol. 39, No. 3, pp. 167-173), investigates the oxide film formation on A508 Grade III / 309L / 308L overlay weld combinations under pressurized water reactor (PWR) conditions. The study is directly relevant to nuclear power plant primary coolant system integrity, where low alloy steel piping with stainless steel overlay welds is a common construction practice to balance mechanical strength with corrosion resistance. The research was supported by the National Natural Science Foundation of China (Grant No. 51771107) and Shanghai municipal programs, underscoring the national strategic importance of this work.
Core Technical Content
Oxygen Content and Oxide Phase Evolution
The central finding is that dissolved oxygen content in high-temperature water fundamentally controls the crystallographic phase of iron oxide formed on the A508 Grade III base metal surface. In oxygenated high-temperature water, the dominant corrosion product is gamma-Fe2O3 (maghemite), whereas in deaerated water, magnetite (Fe3O4) prevails. This distinction is not merely academic; it has direct implications for corrosion rate, film stability, and long-term integrity of the base metal beneath the overlay weld.
| Condition | Base Metal Oxide Phase | Overlay Oxide Characteristic |
|---|---|---|
| Oxygenated HT water | gamma-Fe2O3 (maghemite) | Smaller, less dense spinel particles |
| Deaerated HT water | Fe3O4 (magnetite) | Larger, denser spinel particles |
Overlay Weld Comparison: 309L vs. 308L
The study reveals critical differences between 309L and 308L overlay welds under identical environmental conditions. In deaerated water, the spinel-type oxide particles on both overlay types are larger and more densely packed than in oxygenated water, confirming that reduced oxygen content favors spinel oxide formation. However, 309L produces more and larger oxide particles than 308L under the same conditions.
The most practically significant finding concerns pitting corrosion behavior. Both 309L and 308L overlays were subjected to immersion in both oxygenated and deaerated high-temperature water. Pitting corrosion was observed exclusively on 309L surfaces regardless of oxygen content, while 308L surfaces showed no evident pitting under either condition. The authors attribute this to preferential dissolution of inclusions in the 309L weld metal during high-temperature water exposure, creating localized anodic sites that initiate pitting.
Engineering Significance
For PWR primary coolant piping systems, this finding carries direct engineering consequences. The selection of 309L versus 308L overlay welds should be re-evaluated in applications where long-term exposure to high-temperature water is expected. The pitting susceptibility of 309L, driven by inclusion chemistry, represents a potential failure mechanism that could compromise the protective function of the overlay weld over time.
Process and Standards Analysis
The A508 Grade III / 309L or 308L overlay weld combination is commonly employed in nuclear piping per ASME B31.1 and relevant R-series codes. The overlay weld serves as a corrosion barrier between the carbon or low alloy steel base pipe and the aggressive primary coolant environment. The welding process typically involves GTAW (gas tungsten arc welding) or SAW (submerged arc welding) with multi-pass procedures to ensure adequate dilution control and metallurgical compatibility.
Key process considerations include:
- Preheat and interpass temperature control: A508 Grade III typically requires preheat in the range of 150-250 degrees Celsius to control hydrogen-induced cracking susceptibility in the HAZ.
- Dilution management: The first overlay pass should be applied with a wire composition that provides adequate chromium and nickel content to resist dilution from the base metal. 309L (22-25% Cr, 12-14% Ni) is designed for this purpose, while 308L (18-21% Cr, 8-11% Ni) is more appropriate for subsequent passes or direct stainless-to-stainless welding.
- Post-weld heat treatment (PWHT): Stress relief per ASME Section VIII or B31.1 requirements is essential to reduce residual stresses that could promote stress corrosion cracking (SCC) in the overlay weld.
Key Questions and Reflections
The preferential dissolution of inclusions in 309L weld metal warrants deeper investigation. The inclusion chemistry in 309L is typically richer in manganese and silicon due to its higher nickel content and different solidification behavior compared to 308L. These inclusions may act as cathodic sites relative to the surrounding austenitic matrix, or conversely, may dissolve preferentially to create micro-pits. The distinction between these two mechanisms would have different implications for mitigation strategies.
One question that arises from this work is whether the oxygen content effect on oxide film formation could be exploited as a passive mitigation strategy. If deaerated conditions favor more stable spinel oxide films on the base metal, could controlled oxygen management in the primary coolant system be used to extend the service life of overlay welds? However, this must be balanced against other operational requirements of the reactor system.
The finding that 308L does not exhibit pitting under either oxygen condition is encouraging, but it should not be interpreted as blanket immunity. The study conditions represent specific temperature, chemistry, and exposure duration parameters. Extrapolation to other operating regimes requires additional testing.
Study Insights and Implications
This paper provides valuable empirical data for the nuclear piping industry. The practical recommendation is that 308L overlay welds should be preferred over 309L for applications involving prolonged exposure to high-temperature water, particularly where pitting resistance is a concern. For existing installations with 309L overlays, enhanced inspection protocols focusing on pitting detection may be warranted, particularly at locations where inclusion density is expected to be higher, such as near weld boundaries or in regions of slower cooling rates.
From a metallurgical perspective, this study highlights the importance of inclusion engineering in overlay weld design. Future development of overlay weld consumables for nuclear service should consider inclusion size, shape, and chemistry as design parameters alongside traditional compositional specifications. The interplay between weld metal microstructure, inclusion population, and environmental exposure is a rich area for continued research.
The integration of this knowledge into engineering practice requires updating qualification procedures and inspection criteria for nuclear piping overlay welds. Engineers involved in PWR maintenance and repair should incorporate these findings into their condition assessment methodologies and remaining life analyses.