What qualifies as Stainless Steel?

Dear Matthew:

I'm fascinated by stainless steel, the remarkable metal that doesn't rust...and yet I know so little about it. To qualify as stainless steel, what must it contain? Some stainless steel items are attracted to a magnet and some aren't, which obviously means that some stainless steel contains iron and some doesn't. If you look at ads for stainless cookware, they'll say that they're made of 18/8 or 18/10 stainless. What does this mean? I've seen screws in hardware stores that say they're made variously of #305 and #316 stainless steel. What's the difference? Why do screws use a different type of stainless I.D. number system than stainless cookware?

-- Steely Dan, La Mesa

Whew! We haven't had a love letter like this since our Q about Bismuth, the Wonder Element. So, what's the deal with stainless? Well, it's just a bunch of iron with a smidgen of carbon (that makes it steel), with a handful of chromium thrown in (by law, stainless is no more than 1 percent carbon and no less than 10.5 percent chromium). Chromium makes steel harder and keeps it from corroding, ergo "stainless steel." Add a bit of nickel, and you have a high-quality stainless with a very durable shine. Tweaking the proportions of carbon, chromium, and nickel adapts it to a host of applications. So far, we've cooked up about 300 grades. The cheapest alloys are in the #400-series (low chromium, no nickel). The #300 series contains nickel, so it's better than the #400; #305 and #316 are just two of the more popular grades. Your kitchen sink is probably #316 stainless. The nickel-less #400-grade is the only stainless that will stick to a magnet. Flatware is graded in fractions; high-quality 18/10 stainless has 18 percent chromium, 10 percent nickel. Can't tell you the grade Ford used to make their six experimental stainless-steel-bodied sedans in 1936 or the 1960 T-bird or the '67 Lincoln Continental, but they apparently still look as good as new.