Materials Engineering: A Narrative Pedagogy
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WM_S2014_25

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Welding Metallurgy Spring 2014 Session · 1 section 2 cases · Watch on YouTube ↗ all files
Layer 3 — readable edition

§1. Cleaning aluminum before welding [00:02]

§1.p1

[Continuing from prior discussion of surface contamination.] Something a dog left by the sidewalk, or the goose slipped by the sidewalk. So you have to have good shielding with argon, and you have to clean the surface so you don't have the oxide there to begin with. There are various cleaning techniques.

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§1.p2

You can use sodium hydroxide, which is used a lot because it's not terribly toxic. It can be neutralized in acid and thrown down the drain. Nitric acid, sulfuric chromic — now we're getting into the good stuff. Chromic, that's CrO₃, that's hexavalent chrome, that's Erin Brockovich, that causes cancer. It does a great job of cleaning, by the way. Phosphoric chromic, sulfuric and ferrous sulfate.

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§1.p3

You don't remember it, but right down here, which is now the undergraduate lab, was John Wulff's lab when I was a student. They had a hood right there, and they had a sink, and a big vat of chromic oxide.

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§1.p4

Anybody know why you used to use the chromic oxide? I did this in high school chemistry too — they didn't have as big a vat as we had here. It was a big glass vat of chromic acid, just sat there. When you cleaned your glassware that you'd used in the hood, you're at the sink and you clean it with water and stuff, and you want to get it really squeaky clean. You just take the tongs and dip it in the chromic oxide, drain it in the big bath, and then you drain off the rest — sort of a black liquid — in the sink.

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§1.p5

Well, no one would allow that today. You would not be allowed to just put hexavalent chrome down the sink. If an environmentalist saw one ppm chromic oxide going down the sink, they would have the fire department here immediately, and EPA all over you. But when I was your age, we just dipped it in this great big thing, and most of us didn't even bother putting gloves on. If you spattered some on your skin, you just rinse it down the drain. Things have changed.

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§1.p6

Chromic acid and chromates is a great cleaner for aluminum. And you can see why we usually use sodium hydroxide.

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§1.p7

Let's cut off this and just have a discussion about papers and things.

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Cases referenced

  • MIT undergraduate lab chromic acid disposal practice §1.p3

    Autobiographical. Tom recalls John Wulff's lab — now the undergraduate lab — having an open vat of chromic acid for glassware cleaning, and contrasts mid-twentieth-century practice (no gloves, drain disposal) with present-day environmental and OSHA constraints. Used to motivate the choice of sodium hydroxide as the default aluminum cleaner today.

  • Erin Brockovich hexavalent chromium case §1.p2

    Cultural shorthand only. Tom names the case as a marker for hexavalent-chromium carcinogenicity; no facts of the case discussed.

Layer 2 — cleanup edit
p1 00:02

Something a dog left by the sidewalk, okay, or the goose slipped by the sidewalk okay. And so you have to have good shielding with argon, you have to clean the surface so you don't have the oxide there to begin with. And so there are various cleaning techniques.

p2 00:20

You can use sodium hydroxide, which is used a lot because it's not terribly toxic. It can be neutralized in acid and thrown down the drain basically. Nitric acid, sulf— what's this thing, I can't read it, sulfuric chromic. Okay now we're getting into the good stuff. Okay, chromic, that's CrO3, that's hexavalent chrome, that's Erin Brockovich, that causes cancer okay. So, you know, does a great job of cleaning, by the way. Um, phosphoric chromic okay, sulfuric and ferrous sulfate.

p3 00:56

Um, you don't remember it, but I remember right down here, which is now the undergraduate lab, uh, was John Wulff's lab when I was a student. And they had a hood right there, and they had a sink, and it was a big vat of chromic oxide.

p4 01:15

Anybody know why you used to use the chromic oxide? This, I did this in high school chemistry too. They didn't have as big of that as we had here — was a big vat, big glass vat of chromic acid, just sat there. And when you cleaned your glassware that you'd use in the hood, you're at the sink and you clean it with water and stuff, you want to get it really squeaky clean, you just take the tongs and you dip it in the chromic oxide, and, you know, dip the, you know, drain it in the big bath, the chromic oxide, and then you drain off the rest of the chromic oxide with sort of a black liquid. You just drain it in the sink okay.

p5 01:55

Well, no one would allow that today okay. You would not be allowed to just put hexavalent chrome down the sink. If an environmentalist saw one ppm chromic oxide going down the sink, they would have the fire department here immediately, and EPA all over you okay. But when I was your age, you know, we just dip it in this great big thing, and most of it didn't, most of us didn't even bother put gloves on okay. When we were doing this, so if you spatter someone, your skin, just rinse it down the string, you know. Anyway, so things have changed.

p6 02:27

Uh, but chromie cox— chroma and chromates, okay, chromic acid is a great cleaner for aluminum. And you can see why we usually use sodium hydroxide okay.

p7 02:44

I think let's cut off this and let's just have a discussion about papers and things.