New England Aquarium exterior panel corrosion

Analogy for "bought it in the right condition then transformed it." Stainless steel exterior panels purchased in passivated condition, then mechanically abraded for texture, which removed the passivation.

Sikorsky's chief of rotors testified the day after me, and said, oh, we bought it in the stress-relieved condition. That's sort of like the guy who didn't preheat the weekend before and welded — oh yeah, we did stress relief once. It's like the New England Aquarium: they have stainless steel on the outside structure, supposed to look like fish scales, big sheets of stainless steel, and they bought it in the passivating condition, which grows an oxide skin and keeps it from corroding in salt air almost permanently. Then they went along and did mechanical abrasion to give it some texture. They bought it in the passivating condition, and they scratched off the passivation. Guess what — it's not passivated anymore. So there are three examples of things: well, we bought it in that condition, and then we transformed it, so it's not in that condition anymore. But we bought it in the right condition.

Passivation defeated by post-passivation mechanical abrasion. Mill-passivated 304 panels were ground to create fish-scale texture, which removed the protective chrome oxide. In marine environment (Boston Harbor chlorides), the oxide could not reform. Rust appeared along grinding marks and along bumper scrapes at the loading dock. Tom tested citric acid and Diet Coke (phosphoric acid) as environmentally acceptable cleaning agents.

If you want to go over to the New England Aquarium — how many people have been to the aquarium? What's the outside made out of? Stainless steel. What was the specification? They wanted to make the outside look like fish scales, and they were going to use 304 stainless panels, and they were to be passivated. I read the spec. Passivation means in the mill they took the coils of steel before they had been stamped into fish scales, and they passed them through a bath of nitric acid — about a 5 or 10% solution, not concentrated. It's an oxidizing acid. You clean the stainless steel in the nitric, but you also oxidize it to form the protective oxide. It's now passivated. You can passivate 304 stainless in 15 minutes in nitric acid.

Fish-scale facade specified as passivated stainless but ground for texture, leaving active surface; rust in grinding marks within a year. Tested citric/phosphoric acid/Coke/naval jelly for in-place re-passivation.

If you want to see this, go down to the New England Aquarium. The outside of the New England Aquarium has these little plates that are supposed to look like great big fish scales — part of the design. The specification was that they were supposed to use passivated stainless steel, so it would have marine corrosion protection, right there 20 yards from Boston Harbor. They purchased passivated stainless steel sheet. They formed it, stamped it, and cut it into these big things, and then to give it a little texture they took a grinder and ground away little grinding marks on the surface. When you grind away the surface, you leave active stainless steel underneath the grinding marks. After about a year, you go over there and see right there in the grinding marks a little rust, just following the pits.