📚 Corrosion Reactions | 腐蚀反应
Corrosion is a slow chemical reaction that damages metals when they react with substances in their surroundings. Rusting, the corrosion of iron, is one of the most common examples. In this KS3 chemistry revision guide, you will learn what corrosion is, the conditions needed for rusting, and the main ways to prevent it.
腐蚀是一种缓慢的化学反应，当金属与周围环境中的物质发生反应时就会受到破坏。铁生锈是最常见的例子之一。在这份 KS3 化学复习指南中，你将学习什么是腐蚀、铁生锈所需的条件以及防止腐蚀的主要方法。
1. What Is Corrosion? | 什么是腐蚀？
Corrosion is the gradual wearing away or destruction of a metal caused by chemical reactions with oxygen, water or other chemicals in the environment. During corrosion, the metal atoms lose electrons and form compounds such as oxides, hydroxides or sulfides. This is an oxidation reaction because the metal combines with oxygen or loses electrons.
腐蚀是指金属因与环境中的氧气、水或其他化学物质发生化学反应而逐渐损耗或破坏的过程。在腐蚀过程中，金属原子失去电子，形成氧化物、氢氧化物或硫化物等化合物。这是一种氧化反应，因为金属与氧结合或失去电子。
Corrosion is usually a slow process. It may take months, years or even decades for a structure to be weakened noticeably, which is why chemists describe it as a slow chemical reaction rather than a fast one such as burning. Corrosion begins at the surface of the metal and then works inwards as oxygen and water gradually penetrate deeper into the metal or through cracks in the oxide layer.
腐蚀通常是一个缓慢的过程。一座建筑或构件要明显被削弱，可能需要数月、数年甚至数十年，因此化学家把它称为缓慢的化学反应，而不同于燃烧那样的快速反应。腐蚀从金属表面开始，随着氧气和水逐渐向内部渗透，或通过氧化层中的裂缝进入，腐蚀会不断向内部推进。
Not all corrosion looks the same. Iron forms a reddish-brown flaky solid we call rust. Copper slowly develops a green coating when it reacts with oxygen, water and carbon dioxide in the air, and this green layer is often called patina. Silver tarnishes and turns black when it reacts with sulfur compounds such as hydrogen sulfide in the air.
并不是所有腐蚀的样子都一样。铁生成红棕色的疏松固体，我们称之为铁锈。铜与空气中的氧气、水和二氧化碳缓慢反应后会生成绿色覆盖层，这层绿色物质通常被称为铜绿。银与空气中的硫化氢等含硫化合物反应时会失去光泽、变黑。
2. Corrosion as an Oxidation Reaction | 腐蚀是一种氧化反应
In terms of electrons, oxidation means the loss of electrons. When a metal corrodes, each metal atom loses one or more electrons and becomes a positive ion. For iron, the simplest change can be written as a half equation, which shows only the electrons and the ions involved.
从电子的角度看，氧化就是失去电子。当金属发生腐蚀时，每个金属原子失去一个或多个电子，变成正离子。对铁来说，最简单的变化可以写成一个半反应式，它只显示出参与变化的电子和离子。
Fe → Fe²⁺ + 2e⁻
Because the metal loses electrons it is oxidised, while the oxygen it reacts with gains those electrons and is reduced. This means corrosion is a redox process: oxidation and reduction happen at the same time. The familiar memory aid OIL RIG reminds you that Oxidation Is Loss and Reduction Is Gain of electrons.
因为金属失去电子，所以它被氧化；而与它反应的氧获得这些电子，因此被还原。这意味着腐蚀是一个氧化还原过程：氧化与还原同时发生。常用的记忆口诀 OIL RIG 提醒你，Oxidation Is Loss（氧化是失电子），Reduction Is Gain（还原是得电子）。
Corrosion can also be described using a word equation or a symbol equation. The word equation for rusting shows clearly which reactants are needed, and the symbol equation shows the same change using chemical formulae and balanced numbers of atoms.
腐蚀也可以用文字方程式或符号方程式来描述。铁生锈的文字方程式清楚地显示了需要哪些反应物，符号方程式则用化学式以及配平后的原子数目表示同样的变化。
iron + oxygen + water → hydrated iron(III) oxide
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
The product Fe(OH)₃ gradually loses some of its water and becomes the hydrated iron(III) oxide we call rust, which can be written as Fe₂O₃·xH₂O. In KS3 you only need to know that rust is a compound of iron, oxygen and water, and that the reaction is an oxidation of iron.
产物 Fe(OH)₃ 会逐渐失去部分水，变成我们称为铁锈的水合氧化铁，可写作 Fe₂O₃·xH₂O。在 KS3 阶段，你只需知道铁锈是铁、氧和水组成的化合物，并且这个反应是铁被氧化的过程。
3. Rusting: The Corrosion of Iron | 铁生锈：铁的腐蚀
Rusting is the specific name for the corrosion of iron and steel. Rust is a soft, flaky, orange-brown solid that is much weaker than the iron it came from. Because it is bulky and crumbly, rust does not stay stuck to the surface; it flakes off and exposes fresh iron underneath, which then rusts as well.
铁生锈是铁和钢腐蚀的专门名称。铁锈是一种柔软、疏松、橙棕色的固体，强度远低于原来的铁。由于它体积膨胀且易碎，铁锈不会牢牢附着在表面，而是剥落下来，露出下面新的铁，这部分新铁又会继续生锈。
This flaking is why rusting is so serious. Unlike some protective coatings, rust does not seal the metal off from the air. Instead it keeps removing metal from the inside out, so a thin sheet of iron can eventually become full of holes and a thick beam can become dangerously weak.
正是这种剥落现象使铁生锈变得非常严重。与某些保护性涂层不同，铁锈并不能把金属与空气隔开。相反，它不断由内向外地消耗金属，因此薄铁片最终可能变得千疮百孔，粗大的钢梁也可能变得非常脆弱而危险。
The word equation for rusting is shown below. It reminds you that three reactants are involved, not just one, and this is the key idea that all the prevention methods are built on.
铁生锈的文字方程式如下。它提醒你有三种反应物参与，而不仅仅是一种，这正是所有防锈方法所依据的核心思想。
iron + oxygen + water → hydrated iron(III) oxide (rust)
Rusting causes enormous economic damage every year. Bridges, car bodies, ships, pipelines, railway tracks and reinforcing bars inside concrete all have to be protected, repaired or replaced because of corrosion.
铁生锈每年造成巨大的经济损失。桥梁、汽车车身、船舶、管道、铁轨以及混凝土内部的钢筋，都因腐蚀而需要防护、维修或更换。
4. The Two Conditions Needed for Rusting | 铁生锈所需的两个条件
For iron to rust, two things must be present at the same time: oxygen and water. If either one of them is completely removed, rusting cannot happen at all. This is the single most important fact in this topic and it is examined almost every year.
铁要生锈，必须同时具备两个条件：氧气和水。只要其中任何一个被完全除去，生锈就根本不会发生。这是本主题中最重要的一个事实，几乎每年都会考到。
Notice that the oxygen must come from the air or from dissolved oxygen in water, not from the water molecules themselves. Water that has been boiled for several minutes and then covered with a layer of oil contains almost no dissolved oxygen, so an iron nail in it will not rust.
要注意，这里的氧气必须来自空气或水中溶解的氧气，而不是来自水分子本身。把水煮沸几分钟后，再在表面覆盖一层油，水中几乎不含溶解氧，因此放在其中的铁钉不会生锈。
Also notice that it is not enough to say "air and water". Air is a mixture, and the only gas in air that reacts with iron in this way is oxygen. Examiners expect the precise answer "oxygen and water", and answers such as "air and moisture" may lose marks in a strict mark scheme.
还要注意，只说"空气和水"是不够的。空气是混合物，而空气中只有氧气以这种方式与铁反应。阅卷人期望看到精确答案"氧气和水"，像"空气和湿气"这样的回答在严格的评分标准下可能会失分。
5. The Classic Rusting Experiment | 经典的生锈实验
The standard experiment uses four test tubes, each containing an iron nail, to test the two conditions separately. Only the tube with both oxygen and water present shows rust, and the tube containing salt water shows rust most quickly.
标准的实验使用四支试管，每支都放一枚铁钉，用来分别检验这两个条件。只有同时存在氧气和水的试管中出现铁锈，而装有盐水的试管生锈最快。
| Tube | 试管 | Conditions | 条件 | Result | 结果 | Explanation | 解释 |
|---|---|---|---|
| 1 | Nail in tap water, open to the air | 铁钉置于自来水中，敞口接触空气 | Rusts | 生锈 | Both water and oxygen are present | 水和氧气同时存在 |
| 2 | Nail in boiled water covered with an oil layer | 铁钉置于煮沸过的水中，上面覆盖油层 | No rust | 不生锈 | Water is present but dissolved oxygen has been removed and the oil stops more air dissolving in | 有水，但溶解氧已被赶出，且油层阻止空气再溶入 |
| 3 | Nail in dry air with anhydrous calcium chloride | 铁钉置于干燥空气中，加入无水氯化钙 | No rust | 不生锈 | Oxygen is present but the drying agent has removed the water | 有氧气，但干燥剂除去了水 |
| 4 | Nail in salt water | 铁钉置于盐水中 | Rusts fastest | 生锈最快 | Salt acts as an electrolyte and speeds up the reaction | 盐作为电解质，加快反应速率 |
Anhydrous calcium chloride is used as a drying agent because it absorbs water vapour from the air. You may also see concentrated sulfuric acid used for the same purpose in a closed container, since it also removes water vapour.
无水氯化钙被用作干燥剂，因为它能吸收空气中的水蒸气。你也可能看到在密闭容器中用浓硫酸达到同样的目的，因为它同样可以除去水蒸气。
6. Why Salt and Acid Speed Up Rusting | 为什么盐和酸会加速生锈
Salt does not change what is made during rusting; it only changes how fast the reaction happens. Dissolved salt ions allow charges to move through the water more easily, so the electrons released by the iron can be transferred more quickly to the oxygen. Salt water therefore acts as an electrolyte and rusting speeds up considerably.
盐并不会改变铁生锈的产物，它只改变反应进行的快慢。溶解的盐离子使电荷更容易在水中移动，因此铁释放的电子可以更快地传递给氧气。所以盐水起到电解质的作用，生锈速度会明显加快。
This is why cars near the sea rust faster than cars far inland, and why vehicles in cold countries corrode badly after roads have been spread with rock salt in winter. Sea spray and road salt both supply the same electrolyte effect.
这就是为什么海边的汽车比内陆的汽车生锈更快，也是为什么寒冷国家的车辆在冬季路面撒了岩盐之后腐蚀严重。海雾和路盐提供的是同样的电解质效应。
Acids have a similar effect. Acid rain, formed when sulfur dioxide and nitrogen oxides dissolve in rainwater, also speeds up the corrosion of iron, and it attacks other metals and stone as well. Increasing the temperature also speeds up rusting, because particles move faster and collide more often.
酸也有类似的作用。当二氧化硫和氮氧化物溶于雨水形成酸雨时，也会加速铁的腐蚀，同时还会侵蚀其他金属和石材。升高温度同样会加快生锈，因为粒子运动更快、碰撞更频繁。
7. Why Iron Rusts Faster Than Aluminium and Zinc | 为什么铁比铝和锌腐蚀得更快
Many metals corrode, but iron corrodes faster than some others because its oxide layer is flaky and falls off, exposing fresh metal underneath. Rust is porous and does not stick firmly, so new iron is continually uncovered and the corrosion process never stops.
许多金属都会腐蚀，但铁的腐蚀速度比某些金属快，因为它的氧化层很疏松、容易剥落，从而暴露出下面的新金属。铁锈多孔且附着不牢，因此新的铁不断被暴露出来，腐蚀过程始终不会停止。
Some metals, such as aluminium and zinc, form protective oxide layers that slow down further corrosion. The oxide layer on aluminium is thin, tough and tightly attached to the metal, and it forms again immediately if it is scratched. This self-sealing behaviour means aluminium can look almost unreactive even though it is quite a reactive metal.
像铝和锌这样的金属会形成保护性氧化层，从而减缓进一步的腐蚀。铝表面的氧化层薄而坚韧，紧密附着在金属上，一旦被划伤会立即重新生成。这种自我修复的特性使铝看起来几乎不活泼，尽管它其实是一种相当活泼的金属。
Zinc behaves in the same way. The layer of zinc oxide on the surface protects the zinc underneath, which is why galvanised steel, an iron object coated with zinc, can survive outdoors for many years. Copper is different again: its green patina layer is also protective and slows down further attack.
锌的行为方式相同。表面的氧化锌层保护下面的锌，这正是镀锌钢材（表面覆盖锌的铁制品）能在户外使用多年的原因。铜则又不一样：它的铜绿层同样具有保护作用，可以减缓进一步的侵蚀。
8. Barrier Methods of Prevention | 防腐蚀方法：隔绝法
The simplest way to stop rusting is to keep oxygen and water away from the iron surface. Anything that forms a continuous, unbroken barrier will work, which is why paint, oil, grease, plastic and other metals are all used as coatings.
阻止铁生锈最简单的方法，就是让氧气和水无法接触铁的表面。任何能形成连续、无破损屏障的物质都有效，这就是油漆、油、润滑脂、塑料和其他金属都被用作涂层的原因。
| Method | 方法 | How it protects | 保护原理 | Typical use | 典型用途 |
|---|---|---|
| Painting | 涂漆 | Seals the surface from air and water | 把表面与空气和水隔绝 | Bridges, gates, car bodies | 桥梁、大门、汽车车身 |
| Oiling or greasing | 涂油或涂润滑脂 | Forms a thin water-repelling film | 形成一层薄的斥水膜 | Tools, machine parts, bicycle chains | 工具、机械零件、自行车链条 |
| Plastic coating | 塑料包覆 | Tough, flexible physical barrier | 坚固而有弹性的物理屏障 | Wire fencing, dish racks, garden furniture | 铁丝网、碗碟架、户外家具 |
| Electroplating with chromium or nickel | 镀铬或镀镍 | A thin layer of an unreactive metal is deposited by electrolysis | 用电解方法沉积一层较不活泼的金属 | Taps, bicycle handlebars, decorative fittings | 水龙头、自行车车把、装饰配件 |
The weakness of every barrier method is the same: it only works while the coating stays complete. If paint is scratched, or a plastic layer is cracked, oxygen and water reach the iron and rusting begins at that spot, often spreading underneath the coating where you cannot see it.
所有隔绝法都有一个相同的弱点：只有在涂层保持完整时才有效。如果油漆被划伤，或塑料层开裂，氧气和水就会接触到铁，锈蚀就从该处开始，并常常在看不见的涂层下方蔓延开来。
9. Sacrificial Protection and Galvanising | 牺牲阳极保护与镀锌
A clever alternative is to use a metal that is more reactive than iron. Zinc and magnesium are both more reactive than iron, so when they are in contact with iron in the presence of water and oxygen, they corrode in preference to the iron. The more reactive metal is sacrificed to protect the iron.
另一种巧妙的方法是用比铁更活泼的金属。锌和镁都比铁活泼，因此当它们在有水和氧气的条件下与铁接触时，会优先于铁发生腐蚀。较活泼的金属被"牺牲"以保护铁。
Zn → Zn²⁺ + 2e⁻ (in preference to Fe → Fe²⁺ + 2e⁻)
Galvanising means coating steel with a layer of zinc, usually by dipping the object into molten zinc. The zinc acts as a barrier, but it also protects the iron even when the coating is scratched, because the zinc around the scratch corrodes first. This is the great advantage of galvanising over painting.
镀锌是指用一层锌覆盖钢材，通常是把物件浸入熔融的锌中。锌层起屏障作用，但即使涂层被划伤，它仍能保护铁，因为划痕周围的锌会先腐蚀。这就是镀锌相对于涂漆的最大优势。
Sacrificial protection is also used on a larger scale. Blocks of zinc or magnesium are bolted to the steel hulls of ships, to underground pipelines and to the inside of water tanks. These blocks are replaced at intervals because they gradually corrode away, and they must be more reactive than iron for the method to work.
牺牲阳极保护也被用于更大规模的场合。锌块或镁块被固定在船体的钢壳、地下管道以及水箱内壁上。这些金属块需要定期更换，因为它们会逐渐腐蚀掉；而且为了有效，它们必须比铁更活泼。
Tin plating is the opposite case. Tin is less reactive than iron, so a tin coating protects the iron only while it is unbroken. Once a tin can is scratched deeply, the exposed iron rusts faster than it would alone, because the tin no longer acts as a sacrificial metal.
镀锡则属于相反的情况。锡比铁不活泼，因此锡层只有在完整无缺时才能保护铁。一旦马口铁罐被深度划伤，暴露出来的铁反而会比单独放置时生锈更快，因为此时锡不再起牺牲金属的作用。
10. Alloys, Electroplating and Choosing a Method | 合金、电镀与如何选择防锈方法
Another approach is to change the metal itself by making an alloy. Stainless steel is an alloy of iron with chromium, nickel and carbon. The chromium forms a thin, tough oxide layer on the surface that protects the whole alloy, so stainless steel resists corrosion and is used for cutlery, sinks and surgical instruments.
另一种思路是通过制成合金来改变金属本身。不锈钢是铁与铬、镍、碳组成的合金。铬在表面形成一层薄而坚韧的氧化层，保护整个合金，因此不锈钢耐腐蚀，被用于餐具、水槽和手术器械。
Alloying is usually more expensive than coating, but it is the best choice when the object cannot be repainted or re-greased, for example inside a food factory or in a medical setting. It also avoids the problem of a coating being worn away during use.
合金化通常比涂层更贵，但当物件无法重新刷漆或重新上油时，例如在食品加工厂
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