Comment by Rantenki | Hacker News Reader

Comment by Rantenki | Hacker News Reader

While I am sure there are stylistic reasons for using that color, there is another common reason why you see blue-green colors in paint, especially in older industrial environments: zinc chromate/phosphate corrosion protective coatings. Zinc chromate primer is the color you see on the interior surfaces of some aircraft, to inhibit corrosion. Zinc phosphate is more of a gray in most cases, although varying paint chemistries result in a spectrum between those two, with seafoam nearly smack in the middle.

These are still available today, although the chromate version seems less popular for general use due to toxicity, especially (I assume) in the case of a fire.

I have painted quite a few bits of sheet metal with a sea-foam-ish blue-green/gray paint back in the day (30 years or so ago). I don't recall the manufacturer, but it was a zinc conversion coating in nearly exactly that seafoam color, which has probably stolen at least a few years of my life expectancy. The same company sold other paints in a sickly mustard yellow, and close to fire-engine red, all with slightly different chemistries, I assume for different base metals.

This reminding me of something my granddad demonstrated once.

He used to work on yachts a fair bit and over the years he noticed the fading patterns for different colour paints.

Yellow paint would fade, red paint would fade. But if you mixed them 50/50 into orange, it wouldn't fade. That's why they had so many orange boats in the bay. Figure that one out.

It comes down to each pigment acting as a UV shield for the other. Paint fades because specific wavelengths of light (mostly UV and high-energy visible light) break down the chemical bonds in pigment molecules. The key is that different pigments are vulnerable to different wavelengths.

Yellow pigment absorbs blue and violet light. That's what makes it look yellow, it reflects the longer wavelengths and soaks up the shorter ones. But the UV and violet radiation it's absorbing is also what gradually destroys it.

Red pigment does something similar but across a different band, it absorbs greens and shorter wavelengths, and that absorption is what degrades it over time.

When you mix them 50/50 into orange, each pigment is absorbing the wavelengths that would have destroyed the other one.

I remember reading a long time ago about why barns / shipping containers are dark red. and IIRC it's simply because it's a very similar tone to iron oxide and thus the dye was cheapest to produce.

Not quite right.

Iron oxide is anti-microbial. So a barn painted with it as a pigment will last longer vs mold and such.

It's also why large ships all have that red paint color below the waterline. In that case it's copper oxide, which helps slow barnacle growth and similar.

Back in the age of sail they even went as far as copper sheet cladding to make the wooden hulls last longer. Copper oxide pigment emerged toward the end of the age of sail / beginning of the steamship era as a more practical alternative.

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