一、現(xian)象

  在電解質溶液中（包括潮濕大氣），碳鋼管若與不銹鋼管(guan)相接觸，碳鋼就會迅速產生腐蝕（示意圖見圖8.22），實際例子見圖8.23。后者系城市中用鋼（鐵）絲捆在18-8（304）不銹鋼護欄上所導致的鋼絲的嚴重銹蝕（鋼絲銹蝕還可誘發不銹鋼護欄的腐蝕）。

   圖8.24則系在含NaOH的介質中，石墨熱交換器的碳沉積在316L（022Cr17Ni12Mo2）不銹鋼擋板上而引起的316L不銹鋼管的腐蝕。

二、原因(yin)

   兩(liang)(liang)種(zhong)金(jin)(jin)(jin)屬(shu)在腐蝕介(jie)質(zhi)中，由于二者(zhe)(zhe)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)位(wei)(wei)不同，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)位(wei)(wei)較(jiao)正金(jin)(jin)(jin)屬(shu)為(wei)陰極(ji)，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)位(wei)(wei)較(jiao)負的(de)金(jin)(jin)(jin)屬(shu)為(wei)陽(yang)極(ji)，當兩(liang)(liang)種(zhong)金(jin)(jin)(jin)屬(shu)接(jie)(jie)觸時，它們之間就(jiu)會有電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)產(chan)生，出現電(dian)(dian)(dian)(dian)(dian)(dian)(dian)化學腐蝕（實(shi)際上是形(xing)成(cheng)了宏(hong)觀電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池）。此時作為(wei)陽(yang)極(ji)的(de)金(jin)(jin)(jin)屬(shu)，其(qi)腐蝕速度就(jiu)會提高（加速腐蝕），而(er)(er)作為(wei)陰極(ji)的(de)金(jin)(jin)(jin)屬(shu)其(qi)腐蝕速度就(jiu)會降低（受到(dao)保護(hu)），從而(er)(er)形(xing)成(cheng)異金(jin)(jin)(jin)屬(shu)接(jie)(jie)觸（電(dian)(dian)(dian)(dian)(dian)(dian)(dian)偶）腐蝕。前述兩(liang)(liang)例(li)，前者(zhe)(zhe)舉例(li)表(biao)明，碳鋼的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)位(wei)(wei)遠低于不銹鋼，而(er)(er)后者(zhe)(zhe)則系碳（石墨(mo)）的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)位(wei)(wei)又遠較(jiao)316L不銹鋼為(wei)正，從而(er)(er)產(chan)生了異金(jin)(jin)(jin)屬(shu)接(jie)(jie)觸腐蝕所(suo)致(zhi)。

三、防止措施

   在濕態腐蝕環境中，當不銹鋼管與其他金(jin)屬(shu)相(xiang)接觸(chu)時，要考慮它們之間的(de)電極電位是(shi)否存在顯著的(de)差(cha)異。不要選用與不銹鋼相(xiang)比較電極電位相(xiang)差(cha)過大的(de)金(jin)屬(shu)。