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1、 Corrosion under scale

正好在沉淀物下面或縫隙內(nèi)，溶液中的氧含量是低的，在縫隙的外面大量溶液中的氧含量很高，這就建立了一個(gè)電池，其沉淀物下或縫隙中是陽極而其外面是陰極。含氯化物介質(zhì)的縫隙的內(nèi)部，pH值下降而氯化物濃集。這種酸性氯化物條件導(dǎo)致腐蝕加快并且是自動(dòng)起媒介作用的。

Just under the sediment or in the crevice, the oxygen content in the solution is low, and outside the crevice, the oxygen content in a large number of solutions is very high, which establishes a battery. Under the sediment or in the crevice is the anode, and outside it is the cathode. In the crevice of chloride containing medium, pH value decreases and chloride concentrates. This acidic chloride condition results in accelerated corrosion and acts as an automatic mediator.

接著便發(fā)生了嚴(yán)重的局部腐蝕。這種腐蝕形式的例子：當(dāng)一個(gè)不銹鋼緊固件放置在一塊不銹鋼鋼板上并暴露于含氯化物的水中時(shí)產(chǎn)生?？p隙腐蝕可以在螺栓頭或墊圈作為陽極區(qū)時(shí)發(fā)生。防止沉淀物和結(jié)垢生成或使用高合金含量的材料將有助于減少縫隙腐蝕。

Then severe localized corrosion occurred. An example of this form of corrosion: occurs when a stainless steel fastener is placed on a stainless steel plate and exposed to chloride containing water. Crevice corrosion can occur when the bolt head or gasket is used as the anode area. Preventing the formation of deposits and scales or using materials with high alloy content will help to reduce crevice corrosion.

二、剝落腐蝕

2、 Exfoliation corrosion

在此情況下，金屬表面上形成疏松、片狀的腐蝕層。即使低速流動(dòng)也會(huì)將腐蝕物的疏松層很容易地除去。于是，新的未腐蝕的金屬又被暴露出來，從而將形成許多另外的片狀層。再一次重復(fù)，這些片狀層被很容易地除去并且過程在繼續(xù)進(jìn)行著。使用不易起化學(xué)反應(yīng)的合金可以避免剝落腐蝕。

In this case, a loose and flaky corrosion layer is formed on the metal surface. Even if the flow rate is low, the loose layer of corrosives can be easily removed. As a result, new uncorrosive metal is exposed, and many other lamellar layers are formed. Again, these flakes are easily removed and the process continues. Exfoliation corrosion can be avoided by using alloy which is not easy to react.

三、晶間腐蝕

3、 Intergranular corrosion

出現(xiàn)于某些特殊的合金中，通常當(dāng)它們?cè)诤附踊驘崽幚砥陂g加熱到其敏感溫度區(qū)時(shí)即可能會(huì)發(fā)生晶間腐蝕。當(dāng)諸如某些不銹鋼合金加熱到425～870℃時(shí)，鉻的碳化物即會(huì)在晶粒邊界析出。

Intergranular corrosion occurs in some special alloys when they are heated to their sensitive temperature zone during welding or heat treatment. When some stainless steel alloys are heated to 425-870 ℃, chromium carbides will precipitate at grain boundaries.

導(dǎo)致碳化物附近出現(xiàn)貧鉻區(qū)同時(shí)影響晶界區(qū)的鈍化性。在特殊介質(zhì)中，如硝酸或高溫水中，可能出現(xiàn)低鉻區(qū)的溶蝕現(xiàn)象。晶粒是以一種砂糖似的表面出現(xiàn)的，當(dāng)用一取樣器擦過時(shí)，它們很容易被擦掉。不銹鋼和鎳合金的晶間腐蝕可以通過采用低碳合金、加入碳化物形成元素如鈦或鈮，或利用穩(wěn)定化退火來使之避免。

The results show that the chromium poor zone appears near the carbide and the passivation of the grain boundary zone is affected. In special media, such as nitric acid or high temperature water, the corrosion of low chromium zone may occur. Grains appear as a sugar like surface, which can be easily wiped off by a sampler. Intergranular corrosion of stainless steel and nickel alloy can be avoided by using low carbon alloy, adding carbide forming elements such as titanium or niobium, or stabilizing annealing.