腐蝕鋼板機械性質衰減模型研究：以實船及加速腐蝕鋼板分析

Declining Model of Mechanical Properties for Corroded Steel Plate: An Empirical Study of Real Ships and Accelerated Corrosion Test

中國驗船中心（CR）現有的腐蝕程度檢測方式，大多是使用超音波測厚儀測量鋼板厚度，並在腐蝕厚度達到原先厚度的25%後將其汰換。此種方法雖較簡便，但盲點不少，故期望能針對海洋環境下腐蝕後鋼材的材料強度衰減度進行實驗分析，供設計及檢驗人員參考。在使用定電流極化下進行的加速腐蝕結果顯示，在平均厚度下降5%~35%間的均勻腐蝕對於鋼板的各項機械性質都影響不大，但面臨局部腐蝕時，鋼板的伸長量會隨著蝕孔腐蝕深度上升而大幅下降，當深度達到厚度的20%時，伸長量最多會下降至原先的60%。能得出腐蝕並不影響材料性質，但局部腐蝕造成的表面形貌不平整使得該處應力及應變集中，進而造成伸長量減少。最後本研究透過資料統整提出伸長量衰減率與孔蝕深度的經驗公式，並與舊板實驗數據相互比較。

Ultrasonic thickness measurement is widely used in ship, oil and gas industry for understanding the remaining thickness of metallic substrate in service. For the classification society, detecting the change in thickness of steel plate subjected to corrosion via ultrasonic device is important issue. According to classification rules, the repair or renewal of corroded steel plate should be considered and carried out when remaining thickness of plate is below 75% of original thickness. However, a change in thickness due to non-uniform corrosion could lead to mechanical misjudgment when only applying ultrasonic thickness measurement. This study aims to analyze the change in properties for corroded steel plate used in the marine environment. The results of acceleration corrosion tests show that the Young’s modulus, yielding strength and ultimate tensile strength still remains constant, and the extension rate slightly decreased when general corrosion occurs. By contrast current test, the reducing extension rate of steel plate increased significantly when the pitting depth increased for steel plated suffered from serious localized corrosion. The extension rate would decrease to 60% when the pitting depth up to 20% of original thickness, indicating that uneven surface induced by local corrosion will cause stress concentration, and further decrease in extension rate. Finally, an empirical formula between extension rate and pitting depth was established and compared to these results obtained from the steel plate of real ships.