一、現(xian)象

  在電解質溶液中（包括潮濕大氣），碳鋼管若與不銹(xiu)鋼管相接觸，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(er)、原因

   兩種金屬(shu)(shu)(shu)在腐(fu)(fu)(fu)蝕(shi)(shi)介質中，由于(yu)二者的(de)電(dian)(dian)(dian)(dian)(dian)極電(dian)(dian)(dian)(dian)(dian)位不同，電(dian)(dian)(dian)(dian)(dian)位較(jiao)正金屬(shu)(shu)(shu)為陰(yin)(yin)極，電(dian)(dian)(dian)(dian)(dian)位較(jiao)負(fu)的(de)金屬(shu)(shu)(shu)為陽(yang)極，當兩種金屬(shu)(shu)(shu)接(jie)觸時，它們之間就會(hui)有電(dian)(dian)(dian)(dian)(dian)流(liu)產生，出現電(dian)(dian)(dian)(dian)(dian)化學腐(fu)(fu)(fu)蝕(shi)(shi)（實際上是形成(cheng)了(le)宏觀電(dian)(dian)(dian)(dian)(dian)池）。此(ci)時作為陽(yang)極的(de)金屬(shu)(shu)(shu)，其(qi)腐(fu)(fu)(fu)蝕(shi)(shi)速度就會(hui)提高（加速腐(fu)(fu)(fu)蝕(shi)(shi)），而作為陰(yin)(yin)極的(de)金屬(shu)(shu)(shu)其(qi)腐(fu)(fu)(fu)蝕(shi)(shi)速度就會(hui)降低（受到(dao)保護），從(cong)而形成(cheng)異金屬(shu)(shu)(shu)接(jie)觸（電(dian)(dian)(dian)(dian)(dian)偶）腐(fu)(fu)(fu)蝕(shi)(shi)。前(qian)述兩例(li)，前(qian)者舉(ju)例(li)表明，碳鋼的(de)電(dian)(dian)(dian)(dian)(dian)極電(dian)(dian)(dian)(dian)(dian)位遠(yuan)低于(yu)不銹(xiu)鋼，而后者則系(xi)碳（石(shi)墨）的(de)電(dian)(dian)(dian)(dian)(dian)極電(dian)(dian)(dian)(dian)(dian)位又遠(yuan)較(jiao)316L不銹(xiu)鋼為正，從(cong)而產生了(le)異金屬(shu)(shu)(shu)接(jie)觸腐(fu)(fu)(fu)蝕(shi)(shi)所(suo)致。

三(san)、防止措施

   在(zai)濕態腐蝕環(huan)境中，當不銹鋼管(guan)與其他金屬(shu)相接觸(chu)時，要考慮它(ta)們之間(jian)的電(dian)極(ji)電(dian)位(wei)是否存(cun)在(zai)顯(xian)著(zhu)的差異。不要選用(yong)與不銹鋼相比較(jiao)電(dian)極(ji)電(dian)位(wei)相差過(guo)大的金屬(shu)。