Stress Corrosion Cracking of Fasteners 1

It's not the acid it's the nitrate ion. Nitrates can readily cause SCC of carbon steel under a variety of conditions. The quickest solution is to switch to S/S bolts. There is not much you can do with the existing bolts except replace them if possible.

Give a little more description of the bolts, material, size and the number

Agree with unclesyd on the nitrate cause and (absent any chloride) on switching to SS bolts. Corrosion and Corrosion Control, 3rd Edn., p. 126, describes a bridge failure traced to 12 years exposure of high strength steel (0.7% C) cables to ammonium nitrate-containing rainwater. The National Bureau of Standards found that cable samples failed in tension within 3.5-9.5 months of immersion in 0.01 N ammonium or sodium nitrate at room temperature.

I wouldn't trust coating/plating as a preventative measure on threaded fasteners. Barrier types, e.g., electroless nickel, can be damaged during installation. Sacrificial coating materials Cd & Zn are attacked by ammonia/ammonium ion, and I’m not sure whether they would help, anyway. Best to use resistant material.

Verify that chlorides are not present before switching to austenitic SS bolts. Note that high strength martensitic and PH grades of SS can suffer ammonium nitrate-induced SCC. If austenitic bolts are not available in the required strength, various treatments are available to improve the resistance of mild steel: Cold working to > 50% reduction (e.g., cold heading, with rolled threads) or alloying with > 1% Cr or Mo (while avoiding Ni < 2%). Also, quenching from 900-950[sup]o[/sup]C and tempering at the relatively low temperature of 250[sup]o[/sup]C minimizes susceptibility. ibid., pp. 127-130.