鋅鐵合金鋼片點銹缺陷解析及改善

Analysis and Improvement of Rust Defect on Passivated Galvannealed Steel Sheet

後處理鍍鋅產品表面點銹缺陷為熱浸鍍鋅產線常見的品質問題，其不僅影響產品外觀，更可能衍生下游客戶加工問題。本文針對經化成處理之鋅-鐵合金鋼捲（GA）實際案例進行研討，並藉微結構解析歸納出缺陷成因，繼而調整表面處理劑以改善品質特性，達到客戶加工要求。研究結果歸納如下：（1） SEM分析結果顯示合金化過程造成Zn-Fe合金層最大粗糙度（Rmaximum）過大，導致皮膜於合金層凸處區域覆蓋不足，造成該區域優先形成表面能低的氧化鋅點銹，繼而影響下游塗裝過程漆膜均勻性及火山口缺陷；（2）藉由調整藥劑濃度及組成後，明顯提升皮膜與合金層反應緻密性，鹽水噴霧測試證實改良型GA產品之耐點銹性明顯提升；（3）改良型GA鋼捲經客戶粉體塗裝試用合格後，已正式接單並突破產量12,000噸／年。

Dot rust on the surface of zinc-coated steel products with post treatment is one of the quality problems for continuous galvanizing line. In this paper, the defect case of passivated galvannealed steel (GA) was investigated and resolved by various micro-structural analyses. Results were summarized as follows: (1) Cross sectional morphologies by SEM showed that the larger surface roughness (Rmaximum) of Zn-Fe layer caused the thinner thickness of conversion coating on the peak of the alloy layer and resulted in formation of the tiny rust at these less protective regions. Thus, the poorer wetting ability of the substrate led to the crater defect on paint-coated GAproducts. (2) In order to improve the above defect, the concentration and composition of the solution were adjusted to enhance its reactivity with Zn-Fe layer. Salt spray test approved that corrosion resistance of the modified sample was improved obviously. (3) Finally, modified coils produced by continuous galvanizing line were tried out by customers. Nowadays, production of the modified coils has exceeded 12,000 MT/year.