英語聽力文摘 不銹鋼的秘密

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Why Stainless Steel Does Not Rust But Other Metals Do?

不銹鋼(Stainless Steel)指耐空氣、蒸汽、水等弱腐蝕介質(zhì)和酸、堿、鹽等化學(xué)浸蝕性介質(zhì)腐蝕的鋼，又稱不銹耐酸鋼。實際應(yīng)用中，常將耐弱腐蝕介質(zhì)腐蝕的鋼稱為不銹鋼，而將耐化學(xué)介質(zhì)腐蝕的鋼稱為耐酸鋼。由于兩者在化學(xué)成分上的差異，前者不一定耐化學(xué)介質(zhì)腐蝕，而后者則一般均具有不銹性。不銹鋼的耐蝕性取決于鋼中所含的合金元素。

所有金屬都和大氣中的氧氣進(jìn)行反應(yīng)，在表面形成氧化膜。不幸的是，在普通碳鋼上形成的氧化鐵繼續(xù)進(jìn)行氧化，使銹蝕不斷擴(kuò)大，最終形成孔洞。可以利用油漆或耐氧化的金屬(例如，鋅，鎳和鉻)進(jìn)行電鍍來保護(hù)碳鋼表面，但是，正如人們所知道的那樣，這種保護(hù)僅是一種薄膜。如果保護(hù)層被破壞，下面的鋼便開始銹蝕。

鉻是使不銹鋼獲得耐蝕性的基本元素，當(dāng)鋼中含鉻量達(dá)到12%左右時，鉻與腐蝕介質(zhì)中的氧作用，在鋼表面形成一層很薄的氧化膜( 自鈍化膜)，可阻止鋼的基體進(jìn)一步腐蝕。除鉻外，常用的合金元素還有鎳、鉬、鈦、鈮、銅、氮等，以滿足各種用途對不銹鋼組織和性能的要求。

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.