Stainless Steel and Exhaust: The Whole Story

Not all stainless steel is "stainless." Huh? Read on for the straight dope on the metallurgy of stainless steel.

The GoWesty stainless steel exhaust system is, indeed, made of stainless steel. However, you can expect the system NOT to stay shiny and rust-free looking, and this is why: 

All stainless steel alloys contains iron, chromium, manganese, silicon, carbon, nickel, and molybdenum—all of which can react with oxygen. There are many different stainless steel alloys, but, by definition, all contain at least 10% chromium. Chromium is the key player in keeping stainless steel shiny and aesthetically-pleasing. When stainless steel comes into contact with oxygen (water vapor in the air or liquid water), the chromium reacts with the oxygen to form a protective film on the surface. This stable film limits oxygen access to the metal underneath. The film forms so readily and tightly—even a few atomic layers reduce the rate of corrosion to extremely low levels. In high chromium content stainless steel, the film forms more readily and is invisible. Thus, even though there is a super-thin layer of corrosion, it appears "stainless." Generally speaking, the higher the chromium content, the thinner the protective layer becomes and the more invisible it is—and the shinier the metal tends to stay.

The downside of stainless steel alloys that contain higher levels of chromium is that they are typically more prone to "work hardening." Work hardening is what happens when metal is subjected to repeated stress. A great example of this is what happens to a metal coat hanger when you bend a section of it back and forth. After several cycles, the part that is bending becomes super hard and can't easily be made straight. Eventually, it gets so hard that it just snaps in two. The same thing happens in the exhaust system of a waterboxer. 

Every time the engine is started, run, and shut off, the engine and exhaust system components heat up and cool off; in so doing, they expand and contract. In the case of a waterboxer, the exhaust components are all bolted to the engine in several locations. The engine and exhaust systems do not expand and contract the same amount, so the exhaust system is being worked back and forth, much like what happens in the coat hanger example above. This results in the exhaust system components becoming more brittle over time. By using a stainless steel with lower levels of chromium, our exhaust system components tend to stay more ductile and will last longer—we're talking function over form. 

That's why we chose a stainless steel alloy with a lower chromium content for our exhaust system: It won't stay looking as nice, but it will stay. The surface will not be shiny and beautiful forever—but rest assured that the material under that top layer is unaffected. For this reason, we offer a lifetime warranty on all of our exhaust system components against failure, but not against surface corrosion.

Choosing a stainless steel alloy that does not stay "stainless" looking may not be the best marketing approach—but we're not trying to sell you something shiny and beautiful, only to have it become brittle and break somewhere down the road. From a metallurgical standpoint, we've made the choice that is clearly right for the exhaust on a waterboxer. 

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