鈦製熱交換器冷凝管腐蝕破損原因分析

Failure Analysis of Titanium Heat Exchanger Condenser Tubes

本文探討鈦製熱交換管腐蝕破損原因。一般純鈦在醋酸環境中有很好之抗蝕能力，但某PTA（Purified Terephthalic Acid）廠製程中之兩鈦製冷凝管因不明原因發生異常減薄破損，為了解其破損原因，進行相關之破損原因分析。研究分析方法包括外觀觀察、成分分析、表面XRD分析、截面與表面SEM/EDS分析等。經分析結果顯示E5205冷凝管內壁局部侵蝕呈現高低之浮雕狀侵蝕現象，並有受到流體沖刷。侵蝕區內壁可觀察到針狀之氫化鈦析出，未侵蝕區表面有含Ca、Ti氧化物沉積。E4063內壁呈現灰黑色，沒有明顯沉積；截面內壁可看到明顯氫化物層；內壁表面SEM觀察分析顯示有沖蝕痕跡，並有少量之Fe、Ca元素存在。由分析結果顯示上述兩種純鈦冷凝管皆存在鈦氫化現象，而鈦氫化主要因純鈦腐蝕所導致，並推測腐蝕原因為內壁鐵污染及流速影響所致。

The purpose of this paper is to investigate the failure cause of titanium heat exchangers. In general, commercial pure titanium possesses good corrosion resistance in acetic acid environments. However, abnormal wastage of two titanium condensers was found in a PTA process factory. For failure analysis, some experiments were performed, including inspection examination and analyses of material composition, surface XRD, and SEM/EDS. The results showed that the inner wall surface of E5205 was embossed by corrosion, and eroded by the condensate. In the corroded and non-corroded surface areas, titanium hydride and deposits of Ca and Ti oxide were found, respectively. The inner surface of the condenser E4063 was dark-grey, but on which there were no obvious deposits. Titanium hydride layer was also found in cross section examination; and SEM revealed some erosion evidence and trace amount of Fe, Ca on the inner surface. From the above analysis, it was found that there were titanium hydrides on the surfaces of both titanium condenser tubes. This hydriding phenomenon might be resulted from titanium corrosion, and it was speculated that the corrosion of titanium condensers might come from iron contamination of inner wall surface and flow velocity of the condensate.