Phosphate Rust-Proofing

Add 25 ml phosphoric acid (85%) to 500 ml water in a borosilicate beaker.

Acid into water, never water into acid. Wear goggles and nitrile gloves.

Add 5 g iron filings. Leave until fizzing stops — 20 to 30 minutes.

This is the step people skip and it is the whole patent. Fresh phosphoric acid just dissolves steel. Dissolving iron in it first makes iron phosphate, and it is that near-saturated solution which deposits a coating instead of eating the part.

Scrub the mild steel bar with abrasive paper until bright, degrease it, and handle it by the edges only.

Phosphate grows on steel. It will not grow through rust, scale or a fingerprint.

Bring the bath to a boil on the hot plate — 96–100 °C — and lower the part in.

Hydrogen bubbles come off the steel immediately. Hold at the boil for 20 to 40 minutes.

The bubbling stopping is the finish line. It means the phosphate layer has sealed the surface and the acid can no longer reach iron. The reaction ends itself — that is what makes it a conversion coating rather than etching.

Lift the part out, rinse in hot water, and dry it at once — a hot part dries itself.

The surface is now matt mid-grey and slightly rough. Oil it while warm. The layer is porous and drinks oil into itself; that oil is most of the corrosion protection.

Unoiled phosphate will still rust. This is a primer and an oil reservoir, not a barrier.

Take a second bare steel bar as a control. Leave both outdoors or in a damp place for a week.

Expect: the bare control shows orange rust within days; the phosphated-and-oiled bar stays grey.

For the real lesson, phosphate a third bar and leave it unoiled. It will rust too, just more slowly — which is the honest result, and it tells you the coating is doing its job as a key for oil rather than as armour.

The patent. US 870,937, "Treatment of iron or steel for preventing oxidation or rusting", filed 9 April 1907, granted 12 November 1907, to Thomas Watts Coslett, 17 Jamaica Row, Birmingham. Two corrections to the record: some sources give the inventor as "J. W. Coslett" — the patent prints Thomas Watts Coslett. And it is usually said he "patented it in Britain in 1906"; this US document claims no British priority at all. Treat the 1906 British filing as unconfirmed unless someone produces the number.

What the specification says. The article goes into hot dilute phosphoric acid carrying dissolved iron, the liquor being "evaporated to any desired extent, as for example, to about one seventh of its original volume", leaving a deposit that renders the metal "immune from the deleterious influence of oxidation or rusting". Coslett's iron came from iron filings dissolved in the acid — the trick this blueprint reproduces in step 2.

Coslett did not become Parkerizing. Clark W. Parker bought the rights to the Coslett and Richards processes and founded the Parker Rust-Proof Company in Detroit in 1915. "Parkerizing" is the later manganese-phosphate development sold under Parker's name — Parker was the commercialiser, not the inventor. Crediting him with the invention is the same error as crediting Elkington with electroplating.

Where it went. Zinc phosphate under automotive paint is the reason modern car bodies resist rust perforation; manganese phosphate plus oil is the classic military small-arms finish. Both descend from this bath. It is a rare case where the expired patent is not a museum piece — the chemistry is still running on production lines today.