Metallic alloys for phosphoric acid plants
Chromium is important to increase the corrosion resistance of alloys in phosphoric acid services due to its highly oxidising nature. It is important to consider that the corrosivity of the phosphoric acid depends on the source of the phosphate rock.1,2 Therefore, phosphates
Table 2. Thermal conductivity of graphite and metallic Sanicro 28.3 Material
Thermal conductivity W/m ˚C
Thickness of the tube wall (mm)
Graphite
45
6.35
Wrapped graphite
28
6.35
Sanicro 28
15
2.11
Table 3. Chemical composition of stainless steel used in phosphoric plants.3 Material
UNS
Cr
Ni
Mo
Cu
Others
Sanicro 28
N0828
27
31
3.5
1
-
Alloy 904L
N08904
20
25
4.5
1.5
-
Alloy 20Cb3
N08020
20
34
2.5
3.3
Nb
Alloy 825
N0825
21.5
42
3
2.3
Ti
Alloy G-3
N06985
22
45
6.5
2
W, Co, Nb
Alloy C
N10276
15.5
54
16
-
W, Co
from different groups are often mixed to keep the corrosiveness of the acid at a controlled level. Figure 3 shows the effect of chromium content on the corrosion performance of some alloys using a synthetically produced 'Florida Acid.' 3,4 Table 3 shows the chemical composition of some advanced high-performance alloys (nickel-chromium-molybdenum alloys) that are used in phosphoric acid plants. Sanicro 28 was developed for phosphoric acid and has proven its performance under severe conditions of the phosphoric acid evaporators (steam temperatures of 120 – 130˚C).1,4 In addition, this material has high corrosion resistance in strong acids, resistance to pitting and crevice corrosion, and good weldability. The temperature changes in a different stage of phosphoric acid production affects the corrosion parameters. The influence of the temperature is shown in Figure 4.1 Laboratory tests were carried out in simulated phosphoric acid solutions with impurities such as chlorides and fluorides. Sanicro 28 showed a high level of corrosion resistance. An increase of 10°C results in approximately double the corrosion rate. The importance of a high chromium content is also visible.4,5 This material is a cost-effective solution in contrast to G-type alloys that offer excellent corrosion resistance to phosphoric acid applications but are more expensive with long lead times.1 Figure 5 shows the combined effect of chlorides and fluorides on the corrosion of the high alloyed superaustenitic stainless steel in a superphoshoric acid. The results showed that Sanicro 28 offers acceptable corrosion resistance up to 0.8% fluorides.1,3,4,5
Summary
Sanicro 28 is an austenitic stainless steel that shows high corrosion resistance to this demanding process. It offers easier fabrication and maintenance as compared to graphite heat exchangers which results in lower cost and down time. Graphite tubes have problems during mechanical cleaning as this material is brittle and easily damaged during cleaning or maintenance. In addition, Sanicro 28 has a lower cost than most traditional higher alloyed materials used in phosphoric acid production.
References
Figure 5. Combined effect of chloride ion content and free fluoride ion content on the corrosion resistance at 100°C.3 22 | WORLD FERTILIZER | MAY/JUNE 2023
1. DILLON C.P., MS-7 Material Selection for Phosphoric acid, 1st Edition, 2004: 91 pp. 2. Technical article no. S-TU229-TA-TA. Corrosion resistance and cost-effective alternative to graphite heaters and low corrosion resistance pipes in phosphoric acid production – metallic heaters and pipes in Sanicro 28 (2014). 3. VIERIA B.S., GULLBERG D., PERROT V., Recent Experience with Metallic Heaters for Phosphoric Acid Evaporation. Procedia Engineering 138, 2015: 437-444 pp. 4. AB Sandvik Steel S-52-90, BERGLUD G., TERSMEDEN K. Materials of Construction for Phosphoric Acid Plants Duty. Sandvik AB, Sweden,1999: 1-9 pp. 5. AB Sandvik Steel, S-52-69, Tersmeden, K., Nicolio C., Sweden, 2001:1-8 pp.