首先來談一下金屬腐蝕的方法。一般來說，分為化學腐蝕和電化學腐蝕。

First of all, let's talk about the method of metal corrosion. Generally speaking, it is divided into chemical corrosion and electrochemical corrosion.

化學腐蝕，金屬與接觸到的物質直接發生氧化還原反應而被氧化損耗的過程。例如，化學原料的氯離子與鐵反應生成氯化亞鐵:Cl2+Fe→FeCl2。

Chemical corrosion is a process in which metal is oxidized and lost by direct oxidation-reduction reaction with the contacted substance. For example, the chloride ion of chemical raw materials reacts with iron to form ferrous chloride: Cl2 + Fe → FeCl2.

電化學腐蝕，當金屬被放置在水溶液中或潮濕的大氣中，金屬表面會形成一種微電池，也稱腐蝕電池(其電極習慣上稱陰、陽極)。腐蝕電池的陽極為鐵，而陰極為雜質，鐵一直被犧牲。

Electrochemical corrosion, when the metal is placed in aqueous solution or humid atmosphere, the metal surface will form a kind of micro battery, also known as corrosion battery (its electrode is usually called cathode and anode). The anode of the corrosion battery is iron, while the cathode is impurity. Iron has been sacrificed.

其次來談談金屬腐蝕破壞的方式。主要有以下五種：

Secondly, we will talk about the way of metal corrosion. There are mainly five types:

表面腐蝕：金屬表面的大部分或全部普遍發生的化學反應或電化學反應稱為均勻腐蝕，亦稱全面腐蝕。

Surface corrosion: most or all of the common chemical or electrochemical reactions on the metal surface are called uniform corrosion, also known as comprehensive corrosion.

晶間腐蝕：沿著金屬晶粒間的分界面向內部擴展的腐蝕。主要由于晶粒表面和內部間化學成分的差異以及晶界雜質或內應力的存在。

Intergranular corrosion: corrosion along the boundary between the grains of a metal facing internal growth. It is mainly due to the difference of chemical composition between the surface and the interior of the grain and the existence of impurities or internal stress in the grain boundary.

點腐蝕：簡稱點蝕，通常指具有易鈍化特性的金屬或合金表面在含有Cl離子等有侵蝕性陰離子與氧化劑共存的溶液環境中發生的一類腐蝕。

Pitting corrosion: it is short for pitting corrosion, which usually refers to a kind of corrosion on the surface of metal or alloy with easy passivation characteristics in the solution environment containing corrosive anions such as Cl ions and oxidants.

應力腐蝕：是指在拉應力作用下，金屬在腐蝕介質中引起的破壞。

Stress corrosion: refers to the damage caused by metal in corrosive medium under the action of tensile stress.

腐蝕疲勞：交變載荷與侵蝕性環境的聯合作用，顯著降低構件疲勞性能。

Corrosion fatigue: the combination of alternating load and corrosive environment can significantly reduce the fatigue performance of components.

300系列不銹鋼成分對比

300 series stainless steel composition comparison

304，304L，316，316L這4個牌號，成分對比如下表，可以看出：不銹鋼與碳鋼的成分在微量元素方面差別很大，C，Cr，Ni是主要差別。316系列含Mo 及Ni多于304系列，低碳不銹鋼含碳量只有0.03。

The composition comparison of the four grades of 304304L and 316316316l is shown in the table below. It can be seen that the composition of stainless steel and carbon steel is quite different in terms of trace elements, and C, Cr and Ni are the main differences. 316 series contains more Mo and Ni than 304 series, and the carbon content of low carbon stainless steel is only 0.03.

不銹鋼抗腐蝕的常用方法

Common methods of corrosion resistance of stainless steel

表面鈍化膜防腐

Anti corrosion of surface passivation film

不銹鋼內加入了如Cr， Mo，Ni等合金微量元素。數據表明，當Cr的含量大于12.5%， C對Fe進行合金化處理時，形成了類似于Fe-Cr表面氧化物。這種緊密粘附的富鉻氧化物保護金屬表面，防止進一步地氧化。而且，即使損壞了表層，所暴露出的鋼表面會和大氣反應進行自我修理，重新形成這種氧化物"鈍化膜"，繼續起保護作用。而316系列加入了Mo之后，Mo與Cr協同作用，Mo的氧化物與固溶體形式與Cr的氫氧化物共存于鈍化膜中，進一步提高不銹鋼的點蝕抗力。這也是316比304耐點蝕（含Cl離子）能力強的原因。

Alloy trace elements such as Cr, Mo, Ni are added to stainless steel. The data show that when the content of Cr is more than 12.5%, the surface oxide similar to Fe Cr is formed when Fe is alloyed by C. This tightly adherent chromium rich oxide protects the metal surface from further oxidation. Moreover, even if the surface layer is damaged, the exposed steel surface will react with the atmosphere for self repair, forming the oxide "passivation film" again, and continue to play a protective role. When Mo is added to 316 series, Mo and Cr cooperate. Mo oxide and solid solution form coexist with Cr hydroxide in passivation film, which further improves pitting resistance of stainless steel. This is also the reason why 316 is more resistant to pitting (including Cl ions) than 304.

提高Fe電極電位

Increasing the potential of Fe electrode

不銹鋼加入合金Ni，Cr，Mo等元素，改變電極電位。還是說一下Cr離子，當鋼中鉻量原子數量大于12.5%時，可使Fe的電極電位發生突變，由負電位升到正的電極電位，使得金屬晶體中的電子更加趨于不容易失去，從而有效阻止電化學腐蝕。 316不銹鋼因比304不銹鋼增加了Mo，更進一步拉高了電極電位，因此316系列比304系列更加耐腐蝕。

The electrode potential of stainless steel was changed by adding alloying elements such as Ni, Cr and mo. Or Cr ion, when the number of chromium atoms in steel is more than 12.5%, the electrode potential of Fe will change suddenly, from negative potential to positive electrode potential, making the electrons in metal crystal more difficult to lose, thus effectively preventing electrochemical corrosion. 316 stainless steel has more mo than 304 stainless steel, which further increases the electrode potential, so 316 series is more corrosion-resistant than 304 series.

低含碳量

Low carbon content

含碳量盡可能低。因為碳在不銹鋼晶粒內部的擴散速度大于Cr的擴散速度。在高溫如焊接狀態下，多余的碳就不斷地向奧氏體晶粒邊界擴散，與Cr在晶間形成碳化鉻的化合物，當晶界的鉻的質量分數低到小于12%時，就形成所謂的“貧鉻區”，貧鉻區就會失去耐腐蝕能力，而產生晶間腐蝕，一旦受到應力作用時，即會沿晶界斷裂、強度幾乎完全消失。晶間腐蝕可以分別產生在焊接接頭的熱影響區、焊縫或熔合線上。這也解釋了很多不銹鋼制品總是焊縫處開裂。

The carbon content should be as low as possible. Because the diffusion rate of carbon in stainless steel grain is faster than that of CR. In high temperature such as welding state, the excess carbon will continuously diffuse to the austenite grain boundary, and form chromium carbide compound with Cr in the intergranular. When the mass fraction of chromium in the grain boundary is less than 12%, the so-called "chromium poor area" will be formed. The chromium poor area will lose the corrosion resistance, and intergranular corrosion will occur. Once it is stressed, it will break along the grain boundary, with almost the strength Completely disappeared. Intergranular corrosion can occur in the heat affected zone, weld or fusion line of welded joint. This also explains that many stainless steel products always crack at the weld.

總的來說，

in general,

316(L)系列不銹鋼因添加Mo，故其耐Cl離子腐蝕性、耐電化學腐蝕性都比304（L）優秀；

Because of the addition of Mo, the corrosion resistance of 316 (L) series stainless steel is better than that of 304 (L);

300L低碳的不銹鋼通常只有在特殊情況下如高溫或焊接后才能表現比300系列有明顯的防腐蝕性能，但單純的母材固溶態下防腐性能差別不大。

300L low carbon stainless steel usually has obvious corrosion resistance than 300 series only under special circumstances such as high temperature or welding, but the corrosion resistance of pure base metal in solid solution state is not different.