陰極防蝕電位對鋼材氫脆化影響評估

Evaluation of the Effect of Cathodic Protection Potential on Hydrogen Embrittlement of Steel

離岸鋼構物為防制腐蝕提升設備運轉壽命，目前普遍採用陰極防蝕方式，強度愈高愈可能在陰極保護下可能誘發氫脆現象，反而導致結構物斷裂之風險升高。為協助高強度鋼材之開發與應用於離岸結構物上，乃進行陰極保護下鋼材之氫脆敏感性評估，探討電位對其影響。本研究以ASTM G129為主要試驗參考標準，並以S690Q之母材及銲接件為試驗樣品，採用慢速率拉伸應變方法進行測試，試驗後進行數據分析、破斷面觀察與量測，以評估其脆化傾向。經試驗結果顯示陰極保護電位愈負，其斷面收縮率、塑性伸長率等脆性指標有降低趨勢，顯示陰極保護電位對S690Q氫脆確實有影響。S690Q在-0.95V以上之陰極保護電位其氫脆化傾向小，但在-1.05V保護電位下，發生氫脆化之風險較高。而銲接試棒斷裂處皆發生在熱影響區，與母材試棒相較，具有較高之氫脆化傾向，研判可能因熱影響區生成氫脆敏感性組織，以及銲接殘留應力等原因所造成，因此銲道可能成為陰極保護下優先破裂之處。

For preventing corrosion and promoting the service life of offshore steel structures, the cathodic protection method is generally applied. However, it had been shown that the higher the steel strength, the more likely the hydrogen brittle phenomenon will be induced under the cathodic protection, leading to a higher risk of structural failure. In order to assist the development of high-strength steels and their application to offshore structures, these steels under simulated cathodic protection was evaluated to investigate the effect of the applied cathodic potential on hydrogen embrittlement sensitivity.
With ASTM G129 as the main test reference standard, this study carried out by slow strain rate tensile method and used the base metal and weld metal of S690Q as the test samples. For evaluating their hydrogen embrittlement tendency, data analysis, fracture surface observations and measurement were performed after the test.
The results showed that the more negative the cathodic protection potential, the embrittlement index such as reduction of area ratios and plastic elongation ratios had a more decreasing tendency, it showed that the cathodic protection potential had an effect on S690Q hydrogen embrittlement. As cathodic protection potential was higher than -0.95 V, S690Q had a lower hydrogen embrittlement tendency, but as potential was lower than -1.05 V, the risk of hydrogen embrittlement would be higher. Compared with the base metal specimens, the weld metal showed a higher hydrogen embrittlement tendency and fractures occurred at the heat-affected zones. This phenomena might resulted from hydrogen embrittlement sensitive microstructure and the residual stress of the heat-affected zones generated when welding. The weld metal and HAZ might have a more probability of cracking than base metal under cathodic protection.