【背景介绍】

不锈钢（Stainless Steel）指耐空气、蒸汽、水等弱腐蚀介质和酸、碱、盐等化学浸蚀性介质腐蚀的钢，又称不锈耐酸钢。实际应用中，常将耐弱腐蚀介质腐蚀的钢称为不锈钢，而将耐化学介质腐蚀的钢称为耐酸钢。由于两者在化学成分上的差异，前者不一定耐化学介质腐蚀，而后者则一般均具有不锈性。不锈钢的耐蚀性取决于钢中所含的合金元素。

所有金属都和大气中的氧气进行反应，在表面形成氧化膜。不幸的是，在普通碳钢上形成的氧化铁继续进行氧化，使锈蚀不断扩大，最终形成孔洞。可以利用油漆或耐氧化的金属（例如，锌，镍和铬）进行电镀来保护碳钢表面，但是，正如人们所知道的那样，这种保护仅是一种薄膜。如果保护层被破坏，下面的钢便开始锈蚀。
　　
铬是使不锈钢获得耐蚀性的基本元素，当钢中含铬量达到12％左右时，铬与腐蚀介质中的氧作用，在钢表面形成一层很薄的氧化膜（ 自钝化膜），可阻止钢的基体进一步腐蚀。除铬外，常用的合金元素还有镍、钼、钛、铌、铜、氮等，以满足各种用途对不锈钢组织和性能的要求。

【听力原文】

Our modern world, with its skyscrapers, suspension bridges, and automobiles, relies heavily on steel. If you own an aging car however, you know firsthand that steel has one annoying problem: it rusts. All steel, that is, except stainless steel. What keeps this remarkable material, used commonly in kitchen sinks and cookware, from rusting?

Rust-Proof

Steel’s main ingredient is iron. When steel rusts, its iron combines with oxygen and reverts to iron ore, the raw state from which it came. As this happens, the steel turns brown and begins to crumble.

There are many ways to protect steel from corrosion. Steel is sometimes painted or greased, or coated with a metal that is less likely to rust.

Sacrificial Metal

Sometimes a metal that is MORE likely to rust is attached. Known as a sacrificial metal, this works by drawing the corrosion process away from the steel. For example, bars of zinc are attached to the hulls of some ships. The zinc rusts heavily, but the steel hull stays relatively safe.

Stainless Steel Avoids Rust

The best way to avoid rust is to use stainless steel. Like all steel, stainless steel is mostly iron, but it also contains nickel and chromium.

These are not just a protective coating, but are melted into the steel itself. The mixture must contain at least ten percent chromium, because it’s the chromium that protects stainless steel from corrosion.

What Happens?

What happens is this: Like a sacrificial metal, the chromium rusts first. Unlike iron however, rusting chromium doesn’t crumble apart. Instead, it forms an invisibly thin layer that protects the iron underneath. The nickel in stainless steel helps hold this protective layer of chromium rust in place.

Remember that chromium and nickel are present throughout stainless steel, not just on the surface. Because of this, the microscopic layer will form itself anew, even when the steel is cut or scratched.