一(yi)塊金屬(shu)，表面看來(lai)挺光(guang)滑，可是(shi)在顯微鏡下(xia)就會發現，它的結構很(hen)不均勻，這就為“病從口(kou)(kou)入(ru)”大開了方便之門，因(yin)為金屬(shu)遍體都是(shi)“口(kou)(kou)”，準確一(yi)點(dian)說(shuo)，不是(shi)病從口(kou)(kou)入(ru)，而(er)是(shi)病從“皮”入(ru)了。所以(yi)(yi)金屬(shu)的常(chang)見(jian)病--腐蝕(shi)，一(yi)般都開始(shi)于(yu)“皮膚病”。首先是(shi)金屬(shu)“皮膚潰(kui)瘍”，然后逐漸侵入(ru)到內部。由于(yu)金屬(shu)接觸的物質多(duo)種多(duo)樣，“得病”的條(tiao)件也(ye)各不相同，所以(yi)(yi)金屬(shu)的常(chang)見(jian)病還可以(yi)(yi)分(fen)為許多(duo)種。也(ye)就是(shi)說(shuo)，金屬(shu)腐蝕(shi)可以(yi)(yi)分(fen)為許多(duo)種類。

金屬腐蝕(shi)分類的方(fang)法很多，常(chang)見的一種就(jiu)是(shi)(shi)根據(ju)腐蝕(shi)過程中是(shi)(shi)否有(you)電流產(chan)生，分為化(hua)(hua)學(xue)腐蝕(shi)和電化(hua)(hua)學(xue)腐蝕(shi)兩類。化(hua)(hua)學(xue)腐蝕(shi)一般是(shi)(shi)在(zai)干(gan)燥和高溫的環(huan)境下發(fa)(fa)生的，它是(shi)(shi)金屬的“干(gan)熱病”；電化(hua)(hua)學(xue)腐蝕(shi)是(shi)(shi)在(zai)潮濕(shi)的環(huan)境下發(fa)(fa)生的，它是(shi)(shi)金屬的“風濕(shi)病”。

 化學腐(fu)(fu)蝕是(shi)金屬在不(bu)導電環境(jing)中(zhong)的腐(fu)(fu)蝕，它是(shi)金屬與(yu)某(mou)種物質直接(jie)接(jie)觸時(shi)發(fa)生的化學反(fan)應。在化學腐(fu)(fu)蝕中(zhong)，和金屬接(jie)觸發(fa)生反(fan)應的物質可以(yi)是(shi)氣體(ti)(ti)，也可以(yi)是(shi)液體(ti)(ti)，所以(yi)化學腐(fu)(fu)蝕又分為氣體(ti)(ti)腐(fu)(fu)蝕和液體(ti)(ti)腐(fu)(fu)蝕兩種。

 在高溫下，或(huo)周(zhou)圍(wei)沒有導電物(wu)(wu)質(zhi)的(de)條件下，金屬與空氣(qi)(qi)(qi)或(huo)其他其他發生(sheng)(sheng)(sheng)反應(ying)，表(biao)面(mian)生(sheng)(sheng)(sheng)成(cheng)氧(yang)(yang)化(hua)(hua)物(wu)(wu)或(huo)其他化(hua)(hua)合物(wu)(wu)的(de)現象，稱(cheng)(cheng)為氣(qi)(qi)(qi)體(ti)(ti)腐(fu)蝕。自然界中(zhong)(zhong)，氣(qi)(qi)(qi)體(ti)(ti)腐(fu)蝕的(de)現象也是(shi)很(hen)多。例如，鐵(tie)在空氣(qi)(qi)(qi)中(zhong)(zhong)或(huo)氧(yang)(yang)氣(qi)(qi)(qi)中(zhong)(zhong)加熱(re)時，表(biao)面(mian)會生(sheng)(sheng)(sheng)成(cheng)鐵(tie)的(de)氧(yang)(yang)化(hua)(hua)物(wu)(wu)膜(mo)；鋁(lv)在空氣(qi)(qi)(qi)中(zhong)(zhong)會與氧(yang)(yang)氣(qi)(qi)(qi)反應(ying)，表(biao)面(mian)生(sheng)(sheng)(sheng)成(cheng)一層(ceng)氧(yang)(yang)化(hua)(hua)鋁(lv)薄膜(mo)。像這樣，金屬與周(zhou)圍(wei)的(de)物(wu)(wu)質(zhi)接觸發生(sheng)(sheng)(sheng)反應(ying)生(sheng)(sheng)(sheng)成(cheng)了(le)化(hua)(hua)合物(wu)(wu)，我們就說金屬被腐(fu)蝕了(le)，生(sheng)(sheng)(sheng)成(cheng)的(de)化(hua)(hua)合物(wu)(wu)（膜(mo)）就是(shi)腐(fu)蝕的(de)產物(wu)(wu)（常(chang)稱(cheng)(cheng)作銹）。

金(jin)屬(shu)(shu)(shu)在(zai)(zai)不導電(dian)的(de)(de)(de)(de)液(ye)(ye)體(ti)(ti)里的(de)(de)(de)(de)腐蝕(shi)也是一種(zhong)化(hua)(hua)(hua)學腐蝕(shi)，叫做液(ye)(ye)體(ti)(ti)腐蝕(shi)。銀可(ke)與(yu)(yu)(yu)碘(dian)發生(sheng)液(ye)(ye)體(ti)(ti)腐蝕(shi)。例如把碘(dian)溶(rong)解于乙醚中，制成碘(dian)的(de)(de)(de)(de)乙醚溶(rong)液(ye)(ye)。乙醚是一種(zhong)有機物(wu)，不能導電(dian)。把銀塊(kuai)投(tou)入溶(rong)液(ye)(ye)中，溶(rong)解了的(de)(de)(de)(de)碘(dian)能夠與(yu)(yu)(yu)銀反(fan)應，在(zai)(zai)銀的(de)(de)(de)(de)表面生(sheng)產碘(dian)化(hua)(hua)(hua)銀膜。石(shi)油(you)(you)中往(wang)往(wang)含有硫(liu)(liu)(liu)、硫(liu)(liu)(liu)化(hua)(hua)(hua)氫(qing)及(ji)某些含硫(liu)(liu)(liu)的(de)(de)(de)(de)有機物(wu)，石(shi)油(you)(you)中還含有少(shao)量水，這(zhe)少(shao)量的(de)(de)(de)(de)水一般不能與(yu)(yu)(yu)硫(liu)(liu)(liu)的(de)(de)(de)(de)化(hua)(hua)(hua)合物(wu)形成導電(dian)溶(rong)液(ye)(ye)，但(dan)它卻能加速硫(liu)(liu)(liu)的(de)(de)(de)(de)化(hua)(hua)(hua)合物(wu)與(yu)(yu)(yu)金(jin)屬(shu)(shu)(shu)發生(sheng)化(hua)(hua)(hua)學反(fan)應，成為反(fan)應的(de)(de)(de)(de)催化(hua)(hua)(hua)劑。例如，硫(liu)(liu)(liu)化(hua)(hua)(hua)氫(qing)可(ke)與(yu)(yu)(yu)鐵、鉛(qian)、銅、汞、銀等反(fan)應，生(sheng)成這(zhe)些金(jin)屬(shu)(shu)(shu)的(de)(de)(de)(de)硫(liu)(liu)(liu)化(hua)(hua)(hua)物(wu)，單質硫(liu)(liu)(liu)也可(ke)與(yu)(yu)(yu)銅、汞、銀等金(jin)屬(shu)(shu)(shu)反(fan)應，所以(yi)儲存石(shi)油(you)(you)的(de)(de)(de)(de)容器上部(bu)或蓋子往(wang)往(wang)被硫(liu)(liu)(liu)化(hua)(hua)(hua)氫(qing)腐蝕(shi)。在(zai)(zai)煉油(you)(you)廠，常采(cai)用耐硫(liu)(liu)(liu)化(hua)(hua)(hua)氫(qing)腐蝕(shi)的(de)(de)(de)(de)鋁箔來保護這(zhe)些容器。

化學腐蝕比較簡單，而且只在某些特定條件(jian)下才(cai)會發生(sheng)。在生(sheng)產和(he)日常生(sheng)活中普(pu)遍存在的、破壞最嚴重的是電(dian)化學腐蝕，它是金屬(shu)常患的“風濕(shi)病”。

電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)化腐蝕是指金(jin)(jin)(jin)屬(shu)與電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)解(jie)質溶(rong)液(ye)作用(yong)所發生的腐蝕。它的特(te)點是在腐蝕過程(cheng)中有電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)產(chan)生，這是金(jin)(jin)(jin)屬(shu)表面發生腐蝕電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池作用(yong)的結果(guo)。通常在電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)化腐蝕中規定，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)位較低的金(jin)(jin)(jin)屬(shu)為(wei)(wei)陽極(ji)(ji)(ji)，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)位較高的金(jin)(jin)(jin)屬(shu)為(wei)(wei)陰極(ji)(ji)(ji)。當兩種電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)位不同(tong)(tong)的金(jin)(jin)(jin)屬(shu)相(xiang)接觸，或同(tong)(tong)種金(jin)(jin)(jin)屬(shu)的不同(tong)(tong)的部位具有不同(tong)(tong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)位時，它們(men)侵入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)解(jie)質溶(rong)液(ye)（潮濕氣體、海(hai)水(shui)、酸、堿、鹽的水(shui)溶(rong)液(ye)或土(tu)壤(rang)等）后會(hui)變形(xing)成(cheng)腐蝕電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池。結果(guo)，作為(wei)(wei)陽極(ji)(ji)(ji)的（電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)位低的）金(jin)(jin)(jin)屬(shu)，由(you)于(yu)不斷失去(qu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)子并將自(zi)己(ji)的離子投入溶(rong)液(ye)而被腐蝕，而作為(wei)(wei)陰極(ji)(ji)(ji)的（電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)(ji)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)位高的）金(jin)(jin)(jin)屬(shu)由(you)于(yu)僅起(qi)著傳遞電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)子的作用(yong)，本身沒有發生腐蝕及(ji)其他(ta)變化。