Phosphoric Acid Service Pipe Fittings: Wet Process vs Thermal Process and Why Alloy 20 Is the Baseline Material

Wet-Process Phosphoric Acid (WPPA): The Corrosion Challenge

WPPA is produced by reacting sulphuric acid with phosphate rock (calcium phosphate minerals). The product is a complex mixture: phosphoric acid (30–54% P₂O₅ equivalent), residual sulphuric acid, fluorosilicic acid (H₂SiF₆ — a source of HF), chloride (from chloride in the ore, typically 50–500 ppm Cl⁻), and dissolved heavy metals (cadmium, arsenic, uranium trace quantities). The combination of H₂SO₄, HF, and chloride in a single stream is the reason standard 316L stainless fails: HF attacks the passive film directly; H₂SO₄ creates a reducing acid environment that destabilises passivity; and chloride initiates pitting at the damaged passive film sites. 316L corrodes rapidly in WPPA — measured corrosion rates of 2–5 mm/year are common in 30–54% WPPA at 60–90°C operating temperatures.

Alloy 20 (Carpenter 20 / UNS N08020): The WPPA Baseline

Alloy 20 (nominally 20%Cr, 29%Ni, 2.5%Mo, 3.5%Cu, Nb-stabilised) was developed specifically for sulphuric acid service and has become the baseline material for WPPA pipe fittings. Its performance in WPPA derives from: high nickel (29%) that resists the reducing acid environment; copper (3.5%) that specifically inhibits sulphuric acid corrosion; niobium stabilisation that prevents sensitisation in the HAZ; and sufficient molybdenum (2.5%) for chloride resistance. Typical Alloy 20 corrosion rates in 30% WPPA at 70°C are below 0.05 mm/year — 40–100 times lower than 316L in the same environment. Alloy 20 fittings are manufactured to ASTM B366 Grade WFPN6 (UNS N08020) and are available in standard buttweld geometries per ASME B16.9.

When Hastelloy C-276 Is Required

In the most aggressive WPPA streams — concentrated acid above 50% P₂O₅, at temperatures above 90°C, or with elevated HF/H₂SO₄ impurity levels — Alloy 20 may show unacceptable corrosion rates at specific locations (evaporators, hot concentrated acid headers, HF scrubber connections). Hastelloy C-276 (UNS N10276, 16%Mo, 15%Cr, 4%W) is used in these locations. C-276's high molybdenum content provides resistance to both the oxidising (H₂SO₄) and reducing (HF) components of impure WPPA. C-276 fittings are available in B16.9 geometry but at 3–5× the cost of Alloy 20 — the typical approach is to use C-276 only at the highest-risk locations and Alloy 20 elsewhere in the system.

Thermal Process Phosphoric Acid: A Different Case

Thermally produced phosphoric acid (TPPA) is made by burning elemental phosphorus and dissolving the P₂O₅ in water. It is essentially free of the sulphate, fluoride, and chloride impurities that make WPPA aggressive. TPPA is used in food-grade and pharmaceutical applications where purity is critical. In TPPA service, 316L stainless steel is often acceptable at moderate concentrations and temperatures — the absence of HF and low chloride allows the passive film to remain stable. However, at TPPA concentrations above approximately 85% and temperatures above 80°C, even 316L shows elevated corrosion rates and Alloy 20 may be specified as a precaution.

Rubber-Lined Carbon Steel: The Alternative

For lower-temperature WPPA service (below approximately 60°C), rubber-lined carbon steel fittings are widely used in the fertiliser industry as a cost-effective alternative to Alloy 20. Natural rubber, neoprene, and Hypalon linings provide good resistance to WPPA at ambient to moderate temperatures but are limited by temperature (maximum approximately 60–70°C depending on lining type) and can be damaged by steam cleaning or thermal shock. Rubber-lined fittings are standard for slurry service (gypsum slurry in the filter section of WPPA plants) where the erosive character of the slurry makes metallic fittings impractical regardless of corrosion resistance.