Sulfur and chlorine species in fuel form sulfur trioxide and hydrogen chloride within the combustion products.  At low enough temperatures, sulfur trioxide and water vapor in the flue gas condense as sulfuric acid, causing rapid corrosion. At lower temperatures, hydrochloric acid may condense and promote corrosion of carbon steels and stress corrosion cracking of stainless steels.

During fuel combustion, sulfur oxidizes to form SO₂ and consequently SO₃. The free SO₃ in the flue gas reacts with moisture to form sulfuric acid (H₂SO₄). The concentration of H₂SO₄ depends on the SO₃ and the moisture content; hence the acid dewpoint temperature in the flue gas will vary accordingly. The condensation temperature of H₂SO₄ is approximately 250-300^oF and below. The dewpoint of sulfuric acid depends on the exact concentration of sulfur trioxide in the flue gas but is approximately 280⁰F.   Note that a combination of hydrochloric acid (HCl) and hydrobromic acid (HBr) can condense at a lower temperature and it could be as low as 130^oF.   

Acid dewpoint corrosion produces tube wall thinning that eventually results in ductile rupture of the steel. The external surface exhibits a gouged appearance where the corrosion activity has occurred.  A thin edged fracture surface is produced when the load carrying ability of the steel is exceeded.  Sulfuric acid corrosion on economizers or other carbon steel components is evidenced by general wastage with the sulfuric acid condensation perhaps forming broad, shallow pits.