Anti-corrosion treatment of metal welds

Many metal parts are welded and then rust-protected at the weld zone, usually by chemical conversion treatment - pickling plus phosphating. Two classic failure modes explain why treated weld seams still rust.

Incomplete neutralization: acid from pickling trapped in the seam is not neutralized (or only partially), so the phosphating film forms incompletely - rust develops at the seam and spreads under the paint after a period in service;

Residual salts: strong-acid pickling plus strong-alkaline degreasing leaves reagents that react in the seam but are not rinsed out - after drying, white products appear in the seam and react with airborne moisture after painting, weeping out at the joint.

Degreasing: use a weak-alkaline degreaser - total alkalinity (TA) around 15 points, pH not exceeding 13; strong caustic baths are avoided to prevent residue problems;

Pickling: sulfuric acid can be used but keep the time short; phosphoric acid is preferred - it pickles and contributes to the phosphate film without chloride residue;

Neutralization: after pickling, neutralize with a 1–2% sodium carbonate solution;

Surface conditioning: treat with 1–2% oxalic acid solution before phosphating for a fine, uniform crystalline film;

Phosphating: use a zinc phosphating bath at the upper acidity limit - total acidity (TA) ≥ 35 points, free acidity (FA) ≈ 1.2, accelerator (AC) ≈ 8 points - for a dense, protective phosphate layer;

Rinsing: each washing stage must be thorough, with overflowing water to remove all drag-out.

TA/FA/AC "points" refer to the titration volume (mL) of 0.1 N NaOH per 10 mL bath sample - standard bath-control practice for phosphating lines; confirm limits against the chemical supplier's datasheet.

Reference applicable standards such as GB/T 6807 (phosphating of iron and steel parts) or ISO 9717 (phosphate conversion coatings) when specifying the process.