Phosphoric acid. Synthetic WPA acid

Phosphoric acid, Wet-process acid. From a corrosion point of view, wet process phosphoric acid (WPA) is a rather complicated chemical. The reason is that it usually contains large amounts of impurities, some of which are activating and others will inhibit the corrosion process. The most detrimental impurities are hydrochloric and hydrofluoric and sulphuric acids, while ferric and aluminium ions are beneficial. The following four tables present corrosion test results performed in two series of synthetic wet process acids. In one of the series the detrimental impurities chloride and fluoride have been varied, while the content of the others, iron, aluminium and sulphuric acid, have been kept constant. In the other series the content of iron and aluminium was varied and all the others kept constant. The test acids also contained other compounds in constant quantities. There were: 54% P2O5 , 0.1% SiO2, 0.2% CaO and 0.7% MgO. Due to the rather complex nature of WPA corrosion, the data presented must be regarded as examples only. In critical applications it would therefore be well advised to get additional information from corrosion expertise. The necessary data to determine the risk for corrosion are the composition of the acid and the maximum operation temperature. Phosphoric acid. Synthetic WPA acid Constant concentrations: 1.0% Fe2O3, 1.0% Al2O3, 4.0% H2SO4

Symbol clarification

Phosphoric acid. Synthetic WPA acid
Conc.HF,% 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.6 0.6 0.6 0.6 0.6
Conc.HCl,ppm 200 200 200 600 600 600 1000 1000 1000 200 200 200 600 600
Temp.°C 35 50 85 35 50 85 35 50 85 35 50 85 35 50
Grade or type of alloy:
Carbon steel
13 Cr
Sandvik 1802
Sandvik 3R12
Sandvik 3R60 0 0 0 2 0 0 2
18Cr13Ni3Mo 1)
0 0 0 0 0 0 0 0
17Cr14Ni4Mo 2)
Sandvik 2RK65 ('904L') 0 0 0 2 2 0 0 0
Sanicro 28 0
254 SMO 0 0 0 0 0 0 0 0 0 0 0
654 SMO
Sandvik SAF 2304 2
Sandvik SAF 2205 0 0 0 0 0 0 0 0 0 0
Sandvik SAF 2507 0
Titanium (CP Ti)

1) ASTM 317L, e.g. Sandvik 3R64
2) EN 1.4439, e.g. Sandvik 3R68

Phosphoric acid. Synthetic WPA acid
Conc.HF,% 0.6 0.6 0.6 0.6 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
Conc.HCl,ppm 600 1000 1000 1000 200 200 200 600 600 600 1000 1000 1000
Temp.°C 85 35 50 85 35 50 85 35 50 85 35 50 85
Grade or type of alloy:
Carbon steel
13 Cr
Sandvik 1802
Sandvik 3R12
Sandvik 3R60 2 2 0 0 2 2 2 2
18Cr13Ni3Mo 1)
0 2 0 0 0 0 2
17Cr14Ni4Mo 2)
Sandvik 2RK65 ('904L') 2 2 2 2 2
Sanicro 28 2 2
254 SMO 0 0 0 0 0 0 0
654 SMO
Sandvik SAF 2304 2 2
Sandvik SAF 2205 0 0 2 0 0 0 0 2 2
Sandvik SAF 2507 2 2
Titanium (CP Ti)

1) ASTM 317L, e.g. Sandvik 3R64
2) EN 1.4439, e.g. Sandvik 3R68

Conc.Fe2O3% 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.6 0.6 0.6 0.6 0.6
Conc.Al2O3% 0.2 0.2 0.2 0.6 0.6 0.6 1.0 1.0 1.0 0.2 0.2 0.2 0.6 0.6
Temp. °C 35 50 85 35 50 85 35 50 85 35 50 85 35 50
Carbon steel
13 Cr
Sandvik 1802
Sandvik 3R12
Sandvik 3R60
18Cr13Ni3Mo 1)
17Cr14Ni4Mo 2)
Sandvik 2RK65 ('904L') 2 2 2 2
Sanicro 28 2 0 2 2 1 2 0 0
254 SMO 2 2 2 2 0 0
654 SMO
Sandvik SAF 2304 2 2 2
Sandvik SAF 2205 2 2 2 2 2 2 2
Sandvik SAF 2507 2 0 0
Titanium (CP Ti)

1) ASTM 317L, e.g. Sandvik 3R64
2) EN 1.4439, e.g. Sandvik 3R68

Conc.Fe2O3,% 0.6 0.6 0.6 0.6 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
Conc.Al2O3,% 0.6 1.0 1.0 1.0 0.2 0.2 0.2 0.6 0.6 0.6 1.0 1.0 1.0
Temp.°C 85 35 50 85 35 50 85 35 50 85 35 50 85
Carbon steel
13 Cr
Sandvik 1802
Sandvik 3R12
Sandvik 3R60
18Cr13Ni3Mo 1)
17Cr14Ni4Mo 2)
Sandvik 2RK65 ('904L')
Sanicro 28 0 0 0 0 0 0 0 0 0 0 0 0 0
254 SMO 0 0 0 0 0 0 0 0 0 0 0 0 0
654 SMO
Sandvik SAF 2304 2 2 2
Sandvik SAF 2205 2 2 2 2 2
Sandvik SAF 2507 2 2 2
Titanium (CP Ti)

1) ASTM 317L, e.g. Sandvik 3R64
2) EN 1.4439, e.g. Sandvik 3R68

Symbol clarification

These corrosion tables use a number of symbols, having the following meanings:

SymbolDescription
0 Corrosion rate less than 0.1 mm/year. The material is corrosion proof.
1 Corrosion rate 0.1—1.0 mm/year. The material is not corrosion proof, but useful in certain cases.
2 Corrosion rate over 1.0 mm/year. Serious corrosion. The material is not usable.
p, P Risk (severe risk) of pitting and crevice corrosion.
c, C Risk (Severe risk) of crevice corrosion. Used when there is a risk of localised corrosion only if crevices are present. Under more severe conditions, when there is also a risk of pitting corrosion, the symbols p or P are used instead.
s, S Risk (Severe risk) of stress corrosion cracking.
ig Risk of intergranular corrosion.
BP Boiling solution.
ND No data. (Used only where there are no actual data to estimate the risk of localised corrosion instead of p or s).