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Corrosion

Corrosion can be defined as the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a well-known example of electrochemical corrosion. This is commonly known as rusting. This type of damage typically produces oxide(s) and/or salt(s) of the original metal. Corrosion can also refer to other materials than metals, such as ceramics or polymers. Although in this context, the term degradation is more common.

Most structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area to produce general deterioration. While some efforts to reduce corrosion merely redirect the damage into less visible, less predictable forms, controlled corrosion treatments such as passivation and chromate-conversion will increase a material's corrosion resistance.

Galvanic corrosion

Galvanic corrosion occurs when two different metals electrically contact each other and are immersed in an electrolyte. In order for galvanic corrosion to occur, an electrically conductive path and an ionically conductive path are necessary. This effects a galvanic couple where the more active metal corrodes at an accelerated rate and the more noble metal corrodes at a retarded rate. When immersed, neither metal would normally corrode as quickly without the electrically conductive connection (usually via a wire or direct contact). Galvanic corrosion is often utilised in sacrificial anodes. What type of metal(s) to use is readily determined by following the galvanic series. For example, zinc is often used as a sacrificial anode for steel structures, such as pipelines or docked naval ships. Galvanic corrosion is of major interest to the marine industry and also anywhere water can contact pipes or metal structures.

Factors such as relative size of anode (smaller is generally less desirable), types of metal, and operating conditions (temperature, humidity, salinity, etc.) will affect galvanic corrosion. The surface area ratio of the anode and cathode will directly affect the corrosion rates of the materials.

Galvanic series

In a given sea environment (one standard medium is aerated, room-temperature seawater), one metal will be either more noble or more active than the next, based on how strongly its ions are bound to the surface. Two metals in electrical contact share the same electron gas, so that the tug-of-war at each surface is translated into a competition for free electrons between the two materials. The noble metal will tend to take electrons from the active one, while the electrolyte hosts a flow of ions in the same direction. The resulting mass flow or electrical current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is called a galvanic series, and can be a very useful in predicting and understanding corrosions.

腐蚀
　　(1) [corrode;etch]∶由化学或由化学作用使物体消耗或破坏(例如铁在大气中的生锈)
　　(2) [corrupt]∶使人在坏的思想、坏行为等因素影响下堕落
　　广义的腐蚀指材料与环境间发生的化学或电化学相互作用而导致材料功能受到损伤的现象。
　　狭义的腐蚀是指金属与环境间的物理-化学相互作用，使金属性能发生变化，导致金属，环境及其构成系功能受到损伤的现象。
　　腐蚀的类型 可分为湿腐蚀和干腐蚀两类。湿腐蚀指金属在有水存在下的腐蚀，干腐蚀则指在无液态水存在下的干气体中的腐蚀。由于大气中普遍含有水，化工生产中也经常处理各种水溶液，因此湿腐蚀是最常见的，但高温操作时干腐蚀造成的危害也不容忽视。
　　湿腐蚀 金属在水溶液中的腐蚀是一种电化学反应。在金属表面形成一个阳极和阴极区隔离的腐蚀电池，金属在溶液中失去电子，变成带正电的离子，这是一个氧化过程即阳极过程。与此同时在接触水溶液的金属表面，电子有大量机会被溶液中的某种物质中和，中和电子的过程是还原过程，即阴极过程。常见的阴极过程有氧被还原、氢气释放、氧化剂被还原和贵金属沉积等。
　　随着腐蚀过程的进行，在多数情况下，阴极或阳极过程会受到阻滞而变慢，这个现象称为极化，金属的腐蚀随极化而减缓。
　　干腐蚀 一般指在高温气体中发生的腐蚀，常见的是高温氧化。在高温气体中，金属表面产生一层氧化膜，膜的性质和生长规律决定金属的耐腐蚀性。膜的生长规律可分为直线规律、抛物线规律和对数规律。直线规律的氧化最危险，因为金属失重随时间以恒速上升。抛物线和对数的规律是氧化速度随膜厚增长而下降，较安全，如铝在常温氧化遵循对数规律，几天后膜的生长就停止，因此它有良好的耐大气氧化性。
　　腐蚀的形态 可分为均匀腐蚀和局部腐蚀两种。在化工生产中，后者的危害更严重。
　　均匀腐蚀 腐蚀发生在金属表面的全部或大部，也称全面腐蚀。多数情况下，金属表面会生成保护性的腐蚀产物膜，使腐蚀变慢。有些金属 , 如钢铁在盐酸中 , 不产生膜而迅速溶解。通常用平均腐蚀率（即材料厚度每年损失若干毫米）作为衡量均匀腐蚀的程度，也作为选材的原则 , 一般年腐蚀率小于 1 ～ 1.5mm, 可认为合用（有合理的使用寿命）。
　　局部腐蚀 腐蚀只发生在金属表面的局部。其危害性比均匀腐蚀严重得多，它约占化工机械腐蚀破坏总数的 70 ％ , 而且可能是突发性和灾难性的 , 会引起爆炸、火灾等事故。