Corrosion – Tacking Challenge in Indian Fertiliser Industry
V P Sastry & C V Manian, Technical Committee, NACE International Gateway India Section

Corrosion has continued to be a major concern for manufacturers of phosphatic and nitrogenous fertilisers due to corrosive nature of raw materials used in the production process. Industry runs high risk of encountering un-foreseen downtimes due to damage caused by corrosion and result in heavy losses to the fertiliser manufacturers. The authors briefly review typical forms of corrosion observed in fertiliser plant and recommend measures to improve overall plant reliability.

India's agriculture industry is a major consumer of phosphatic fertilisers viz, single and triple super-phoshates; and nitrogenous fertilisers which include - ammonium sulphate, ammonium nitrate, di-ammonium phosphate, calcium nitrite and urea.

Phosphate rock is the primary source of raw material to produce phosphatic grades. For nitrogenous fertilisers, hydrogen and nitrogen are the main raw materials for ammonia production. Typically, hydrogen (H) is generated by steam reforming of natural gas, while nitrogen is obtained through air liquefaction. The production process involves use of acids like phosphoric acid, nitric acid and sulphuric acid that are produced on-site (Figure 1).

Corrosion in urea manufacturing plants
Corrosion is determined by temperature, process components and concentration of dissolved oxygen. Presence of contaminants may accelerate corrosion. Use of stainless steel (SS) lined carbon steel vessels in HP synthesis section and leak detection units (1, 2) can help mitigate the impact of chemicals on the process unit. Some of the typical forms of corrosion that occur in urea plant include
  • Active corrosion is one of the most common forms of corrosion that can be prevented by exposing SS to carbamate solutions and passivated by the design amount of oxygen, else SS suffers severe thinning thus reducing longevity of the equipment
  • Erosion corrosion
  • Inter Granular Corrosion (IGC)is due to oxidizing action of oxygen containing urea carbamate solution, a low NH3/CO2 ratio and segregation of impurities in sensitized SS.
  • Any chloride ingress in the shell side of the stripper/carbamate condenser will initiate Stress Corrosion Cracking (SCC) on austenitic SS tubes, whereas duplex SS material resists SCC.
  • Galvanic corrosion occurs in intercooler and after cooler of CO2 gas cleaning circuit (3), which in some cases have resulted in shutdown of the plant within six months. In one of the cases, AISI type 304 SS parts - sealing strips, fins, demisters and the shell that were in contact with the duplex SS tubes suffered galvanic corrosion. Remedies reported are 304 SS for all parts of inter and after coolers; cool scrubber exit CO2 gas to condense water and increase 0.8 percent oxygen in CO2 gas before entry into the intercooler for an effective passive film on SS.

Acid Attacks
Phosphoric Acid(4): Level of impurities like fluorides and chlorides in phosphate rock decides the extent of attack downstream processes. Some of the reported failures include SCC and pitting/ crevice corrosion, thinning and leaks of phosphoric acid tanks and acid loading pipe cracking. Techniques such as 'redox adjustment' or 'Mg addition' are reported to reduce corrosion. The process water associated with by-product phosphor-gypsum is acidic and corrosive.

Nitro-phosphoric Acid(4): Used in manufacture of nitrate containing compound fertilisers. Temperature control to 700C and monitoring is critical to minimize corrosion in reacting rock phosphate in HNO3.

Nitric Acid: Corrosion protection is through suitable austenitic SS.

Aqueous Amine Solutions: Mono ethanolamine, di-ethanolamine or hot potassium carbonate solutions used for absorption of CO2 in the process are corrosive, inhibited by V2O5.

Urea Ammonium Nitrate (UAN)(5): Acidic nature of UAN makes it highly corrosive to CS, and results in 'surface corrosion' when the ammonia levels are low. Accumulated corrosion deposits lead to crevice formation between/and eventually pitting. It is very important to control the level of ammonia when the surface is wet with UAN as excess ammonia of the order of 100 to 2000 ppm helps to mitigate the corrosion. Some of the control practices include:
  • Cleaning at least once in two years; recirculation of sludge during in -between period
  • Use sloped bottom tank for removal of sludge
    1. Monitor and maintain neutral pH, with ammonia levels >100ppm
    2. Salt out condition causes corrosion if AN/Urea ratio is not between 1.2 to 1.4
    3. Use filmer or passivator type of corrosion inhibitor
    4. Use coatings, linings , SS or plastics MIC
Microbial Induced Corrosion (MIC): This is a perennial problem in fertiliser waste waters. Heat exchanger tubes show MIC leading to pitting and fouling (6, 7). DAP-NPK in the effluents serve as nutrients for a) Fungus Aspergillus to produce corrosive organic acids, b) sulphates and low DO (dissolved oxygen), favorable to H2S production by Desulphovibrio desulphuricans, Clostridium perfringens, thus resulting in MIC.

Potassium and chlorine deposits: Fertiliser manufacturers are now moving to use of biomass as an alternative source to derive the H requirement. Gasification of straw is one such source of H but poses the problem of corrosion due to the threat of high K and Cl deposits(8).

Storage & Handling
Corrosion in storage of fertilizer solutions can due to three main factors - decomposition or induced reaction to produce aggressive substances such as NH3 or H2S, presence of chloride ions - including K or NH4Cl, or if acidic conditions prevail. Generally, in dry conditions, no corrosion is reported. Initially, moisture ingress results in caking, adding abrasive properties, in addition to corrosion.

MOC like CS have limited resistance to corrosion while galvanized steels, aluminum and SS can withstand corrosion to a greater extent as compared to CS. Paints and coatings can generally improve equipment life. Tables 1, 2 & 3 describe the impact of corrosion on different materials of construction (MOC).

Improving Plant Reliability
Current fertiliser production plants are very complex with individual units operating at various temperatures and pressures as per the licensor technology employed by the end-user. The plant availability and reliability is enhanced substantially by adopting following modern techniques:
  • HAZOP study: It is a design stage activity and continued at each stage when modifications, affecting the plant performance, are incorporated. It is a structured review of the Piping and Instrumentation Diagrams(P & ID) with the project group to ensure that adequate controls, instrumentation and interlocks are in place to take care of any process upsets,
  • Safety Integrity Level(SIL): Analysis of control system should be mandated to ensure that the critical loops in the plant are in conformity with International Safety Standards IEC 61508 & IEC 61511and to verify that the plant's safety functions meet the SIL requirements and to recommend possible mitigations if SIL standards are not complied with.
  • Emergency shutdown (ESD) and proper start up of the plant. All plant personnel should be drilled into ESD procedures.
  • Provision of Distributed Control System (DCS) and Advanced Process Control System (APC) should be provided to take care of inherent flexibility required in process operations, because of changes in quality and availability of feed stock, changes in process conditions.
  • Selection of trustworthy and reputable vendors, who will work with the industry at all stages of plant change implementation, is critical.
  • Provision for improving plant performance by incorporating corrosion monitoring techniques, which can form the basis for Residual Life Analysis and Aging Management programs. Typical examples of such activities will include provision of corrosion coupons, NDT testing programs, tested and reliable models to extract information out of corrosion monitoring devices .
  • Complete understanding on the effect of materials selection on plant corrosion and hence its operation. Typical examples for a reformer section, urea plant and characteristics of FRP resin are listed here and comparison of MOC used in urea plant (Table 4).
    1. Primary Reformer performance has been improved by change of tube metallurgy from HK-40 to Micro-alloys like Manurite 36M, Paralloy H39W.
    2. Urea Stripper has undergone up-gradation from Ti tubes to Zr tubes and to Sandvik 2RE69.
  • FRP: In 'wet process' H3PO4 and H2SO4, typical MOC include high Ni alloy , rubber-lined CS, acid brick-lined CS and resin coated CS. Choice is based on life expectancy and cost (Table 5). FRP with epoxy vinyl ester resin seems to be a suitable material for some equipment and piping and for abrasion resistant applications (gypsum slurry). Typical laminate is shown in Figure 2
Adopting Risk Based Inspection (RBI) / proper Corrosion Monitoring Programs can lead to significant improvement in plant reliability and reduce annual routine shutdowns.

  • Fertiliser manufacturers should develop methodology to maintain and optimise inspection programs for high corrosion risk to fertiliser plant equipment.
  • There is a need to compile the available data on corrosion in fertiliser industry since there is very limited data available.
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