How did my knife get pitted?
December 7, 2013 4:19 PM   Subscribe

Decent paring knife emerges from bleach-soap-water presoak with pitting corrosion. WTF?

Today as I was loading our dishwasher I noticed one of my semi-good chef's knives (a five-inch paring knife) had some odd black spots on the blade near the haft. After scrubbing it carefully I realized that the spots were pitting corrosion.

I worked as a dishwasher years ago and an important technique for prepping the dishes and silverware for washing was the presoak, which helps prevent glutinous food waste from drying onto the surface of the dishes. I have adapted this in our house to be a small plastic tub kept in the sink with water, soap, and a splash of bleach in it. When a utensil is moved to the sink after use, if it's small, it goes into the presoak, used part down.

I had previously noted an increasing incidence of slight rust marks on some items coming out of the dishwasher and eventually realized it was due to a thrift-store table knife developing a crack in its presumably nickel-stainless chrome plate coating; the interior of the blade was rusting and gradually peeling the plate back from the cracked area. I didn't think about it much beyond noting it and decoding to junk it sometime.

This cracked knife was in the presoak along with the newly-pitted knife for about 18 hours, I guess. The linked wikipedia article notes that the presence of chlorides in solution can provoke an anodizing reaction that leads to this type of corrosion; I assume that the bleach in my presoak is the chloride needed.

Given that we have evidence that the corrosion has occurred in these circumstances, the short answer would be "don't do that," I think.

My specific question, though, is am I correct in fingering that cracked and rusty knife as the catalyst? I have been doing this presoak thing for years and had not ever noted pitting as a consequence previously.
posted by mwhybark to Science & Nature (13 answers total) 1 user marked this as a favorite
How long was the paring knife in your presoak solution --- hours, days, even weeks?

I ask because the only time I've seen pitting on tableware was a stainless-steel serving spoon one of my sisters (a slob) had left in a soap & water solution for what she admitted was 'about two weeks, I think', but was probably more like a month or even two.
posted by easily confused at 4:33 PM on December 7, 2013

Bleach is extremely corrosive. If your presoak really had bleach in it, I wouldn't be surprised by corrosion at all.
posted by Chocolate Pickle at 5:04 PM on December 7, 2013 [3 favorites]

Bleach is a no no with high carbon or austentic steels.

If you need to pre-soak things like that you might hit a homebrew shop and get some PBW - a very powerful surfactant that will remove gunk without harming the metal.
posted by BrooksCooper at 6:03 PM on December 7, 2013 [4 favorites]

How long was the paring knife in your presoak solution --- hours, days, even weeks?

about 18 hours, as previously noted.
posted by mwhybark at 6:23 PM on December 7, 2013

Former cook, high end. I wash my knives in a dishwasher twice a year. Once in June / July and once at Thanksgiving. They do not go through a drying cycle. Instead, my normal routine is to clean my knife in the sink first. I clean it multiple times during use. In the kitchen, a good knife is one of the most important tools. I keep my knife clean and free of debris. While I generally dry it immediately, maybe once a week, the knife does dry in the dish rack. My knife never lives in a block, and is generally honed, though it does get sharpened on a stone with food-grade mineral oil about every two weeks. When I cooked, I sharpened every day, with a once a week major overhaul on my knives.

Even stainless steel rusts and pits. Moisture leads to damage to the surface. Leaving a wet knife in a kitchen block, allows the knife to spend extra time in a moist environment. Overheating a knife in a dishwasher expands the material, allowing easier access for moisture to create a foothold on your knife. Similarly, I'd say chlorine exacerbates it as well; however, if you're using the standard sanitation restaurant technique, its dilute enough that corrosion is extremely rare and generally only affects pots and pans with significant marring. It doesn't help; however, it more than likely exacerbated a situation created by the dishwasher and your habits of soaking your knives instead of immediately cleaning them.

You'll need to move to oiling your knife regularly. Ideally use a high quality food grade mineral oil; however, in the short term a little olive oil will protect the blade from further damage and oxidation.
posted by Nanukthedog at 6:23 PM on December 7, 2013 [3 favorites]

Brooks, I love that idea. I'm after an antibacterial solution that I can leave set for varying periods. The non-swap resolution is more or less, as noted, don’t do that (which would equate to making sure not to put aluminum and carbon steel utensils in the presoak).

Nanuk, much appreciated. I don't have anything high-end enough to worry about running thru the dishwasher, but the best-practices info is right on.
posted by mwhybark at 6:26 PM on December 7, 2013

and I should note I don't generally measure the bleach as it goes into the tub (a 2-pint cottage cheese container) and that when I set up the solution my wrist failed me and more than usual went into the bucket. A normal amount is a splash, less than a capful. This might have been two capfuls.
posted by mwhybark at 6:34 PM on December 7, 2013

Even a small amount of bleach could do it. Bleach is hypochlorite, which readily transforms into chloride and an oxygen radical.

Oxygen radicals are very active. That's why bleach can burn you if it stays on your skin for very long. It's not the same kind of thing as an acid but it's just as dangerous.
posted by Chocolate Pickle at 9:58 PM on December 7, 2013 [1 favorite]

I agree with the consensus that it was the bleach. I found this out the hard way when I attempted to sanitize an old double-edged safety razor I bought on eBay.

It's fine to presoak, but I don't think the bleach servers any purpose in it. It doesn't make sense to sanitize dishes before you wash them. I'm a licensed food handler and restaurant owner and the sanitizing part of the standard three compartment sink is last. I suppose you could keep using soapy water in the soak, but just regular water should also be fine.
posted by Tanizaki at 10:13 PM on December 7, 2013

Yeah, bleach is a decent kitchen cleaner, but it is very, very hard on steel.
posted by xedrik at 10:16 PM on December 7, 2013

I worked as a dishwasher years ago and an important technique for prepping the dishes and silverware for washing was the presoak, which helps prevent glutinous food waste from drying onto the surface of the dishes.

It might help to re-think this in a more goal-oriented manner; the easiest and shortest route to a clean knife.

First, IF you soak it, you need only water and dishwashing detergent. Your kitchenware is not a bathroom floor.

Second, 18 hours is unnecessarily long.

Third, as Nanukthedog says, kitchen knives, no matter how non-high-end, are best rinsed off under the hot tap with a little dish soap directly after use, dried and put back to wherever they live. No glutinous gunk should ever get the chance to even dry onto your knives. This is safest (you won't ever accidentally grab a bunch of forks and spoons And There Was That Knife), your knife stays sharp longer if it doesn't routinely bangs against other tools, and most of all: nothing else is needed to keep it clean - it's almost no work at all.
posted by Namlit at 10:53 PM on December 7, 2013 [1 favorite]

For a while there my wife would yell at me when I did dishes because I'd do the qualitative analysis tripple rinse and then hold it up to make sure the water was sheeting off of it evenly and not "breaking" around some spec. That said, _I_ think an 18 hour pre-soak is pretty over the top.

With bleach, well, NaClO + Fe → FeO + NaCl, which makes it pretty much NOT the thing to use on steel.

You're unlikely to have bacterial growth on clean dry dishes anyway so I think you're being way over the top here.
posted by Kid Charlemagne at 11:48 PM on December 7, 2013 [1 favorite]

Late to this particular question, but I'd like to point out that PBW, and other percarbonate based cleaners will pit aluminum with extended contact.
posted by nulledge at 5:40 AM on December 10, 2013

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