Feature

MOC for Corrosive Acid Plants

Posted on 10 February, 2010 | Tags: Engineering Materials

Every year industry incurs huge losses due to corrosion. Acids though highly corrosive in nature, are an important part of manufacturing process in the Fertilizer Industry. Selection of right Material of Construction (MOC) is an important criteria to ensure longer plant life.

Industrial growth is a clear indicator of growth of a nation. The progress of a country depends directly up on the development of its Chemical, Petrochemical, Heavy Chemicals and Fertilizers Industries. Fertilizers have a vital role in agricultural production. The growing demand of food products to meet the demand of growing population will offer immense scope for growth of Fertilizer Industry across the globe.
Acids, are that highly corrosive chemicals are utilized by most of the process industries. Refineries, Petrochemicals, Fertilizers, Heavy Chemicals, Pharmaceutical and Fine Chemicals etc use acids either as raw materials or intermediates to support the chemicals for operation.
Phosphoric acid (H3PO4), Sulphuric acid (H2SO4), Nitric acid (HNO3) and Hydrochloric acid (HCl) are some of the acids that are mostly used as raw materials for to manufacture Phosphate or Complex Fertilizers. Since these acids are highly corrosive in nature, selection of right type of materials of construction is of utmost importance for smooth and safe plant operations.
The problems due to corrosion are very complex in nature because these result in change in chemical compositions. Overflow and leakage of various chemicals in the form of liquids or vapors corrodes the process equipments, pipelines, tanks and vessels etc. This can result in premature failure of equipment and lead to financial losses and irreversible damage to the environment. The losses due to corrosion are estimated at around Rs 35000 crores. 
The equipments like pumps, vessels and tanks etc that are used for acid plants for H3PO4, H2SO4, HNO3 and HCl generally get affected because of corrosion that damages the plant, which eventually results in premature failures and heavy financial losses.

Types of Corrosions in Acid Plants
There are 7 main forms of corrosion that take place in acidic, corrosive and erosive mediums. These affect the life of pumps and piping system along with other system components.

• General (Uniform) Corrosion is most common that can be identified as the uniform removal of metal by reaction with the fluid or environment.
• Crevice (Pit) Corrosion occurs because of solid deposition in the areas that are not much exposed to the fluid flow. These include spaces such as in the threads, under the bolt heads and on the seal surfaces. The spaces where the vapor gets trapped corrode because of pitting.
• Erosion Corrosion results from general corrosion and mechanical abrasion. A corrosive fluid with solid particles in suspension and high fluid velocities that are experienced within pump metering heads and valves can lead to this type of corrosion in the acid plants.
• Cavitations corrosion is a form of erosion-corrosion. It occurs because of poor Net Positive Suction Head (NPSH) conditions that result in formation of bubbles in the fluid. These bubbles break with enough force thus allowing the chemicals attack subsurface material. The bubbles can collapse with extreme force and actually break away small particles of metal.
• Intergranular Corrosion is the electrochemical attack on the grain of metal. This occurs where welding or stress relieving results in change of material composition.
• Stress corrosion occurs when metal is under tensile stress at temperatures well above ambient. Often, embrittlement and cracking occur without evidence of severe corrosion.
• Galvanic Corrosion usually results because of selection of dissimilar metals in a system that allows accelerated corrosion of less noble metal because of electrochemical attack. Unknowingly, the manufacturer while pump selection invites this potential problem.

Although, some of these problems may not be as critical to pump and piping system design as these are to plant system design, but these must be factored for safe plant operation.

Intergranular corrosion, Stress corrosion and Galvanic corrosion relate equally to the system as well as pump design. Corrosion may start to occur to the process equipments at any of the stages like welding and stress relieving, assembling piping and other system components or even while dissimilar metals are introduced into the system.
Besides corrosion, there are several other important factors that must be factored in at the time of deciding on MOC for specific service and use. This is an extremely challenging job for metallurgists and engineers, as they need to balance several technical as well as economical factors.

Selection Criteria for MOC
Points to be considered while deciding on MOC

• Corrosive, Physical, Mechanical, Chemical and Physical Properties and Appearance of the Material
• Ability for Fabrication by Formation, Welding and Machining
• Compatibility with the already existing equipments
• Ease of maintenance
• Specification coverage
• Total expected life of the plant
• Estimated service life of the material
• Cost and availability of the material
• Availability and cost of skilled labor for maintenance
• Size of the plant
• Type and Number of process streams
• Cost of fabrication
• Need for further testing etc
• Costs of maintenance and inspection
• Methods of corrosion control
• Prior service experience
• Availability of materials at the location
• Analysis of return on Investments (ROI)

Selection of MOC for Phosphoric Acid and Phosphate Fertilizer Plants is very difficult because of unpredictable nature of Rock Phosphate and its impurities like iron, aluminum, sodium, potassium magnesium, fluorine, chlorides and silica etc. Silica has an indirect effect on corrosion as much as it reacts with Hydrogen Fluoride (HF) to form Silicon tetra fluoride (SiF4). Low silica content allows HF to remain in the acidulation slurry product and make it more corrosive for the equipments as well as the pipelines.
Rock phosphate is available in different grades with low and high content of phosphates are available across various parts of the world. There are approximately 60 grades of Rock phosphates from deposits of 24 countries in the world Typical chemical analysis of high and low grades of Rock phosphates are listed in Table 1
Almost all the industrially manufactured Phosphoric Acid is now produced directly from Rock phosphate (ore) using Wet Process i.e. Acidulation with H2SO4. During the process, most of the impurities of Rock Phosphate are dissolved in the solution and these remain in the final product, which is Phosphoric Acid solution.
Corrosive nature of slurry depends on the temperature, content of free H2SO4 and the velocity of movement of the slurry particles inside the vessel. The reaction slurry contains highly abrasive Gypsum crystals, approximately 40 percent by weight. The presence of these crystals in the slurry further aggravates erosion thus making it more difficult to select right type of MOC. Corrosive impurities present in Rock Phosphate, velocity of slurry flow and temperature considerably have an impact on the Plant life. Some of the impurities present in Rock Phosphate are Chlorine (Cl), Fluorine (F), H2SO4 and Salts of Calcium (Ca), Potassium (K), Magnesium (Mg) and Sodium (Na).
Chlorine is present in Rock Phosphate as mineral chloride. This also comes from the water used for Rock washing and beneficiation. At the time of Acidulation, these chlorides form HCl, which is highly corrosive and destroys the passive film formed on the metal surface. It also allows Pitting corrosion to occur on the metal surface. Observations have revealed an exponential relation between Rate of corrosion caused by Cl and Temperature.
Around 10-14 percent of Fluorine is present in all the Rock Phosphates in relation to P2O5. During Acidulation, this forms HF, which reacts with reactive silica present in Rock Phosphate and forms Hydro-flosilicic Acid (H2S1F6) that is non corrosive in nature. To some extent, Aluminum as MOC helps to reduce corrosion by fixing the fluorine as Aluminum fluosilicate. If Fluorine to Silica ratio F/SiO2 is more than 1.8, then HF remains as a free acid and results in excessive corrosion.
Presence of salts of Ca, Na, K and Mg leads to scale formation and does not contribute to corrosion. This reduces corrosion by reacting with HF to form fluoluminates or fluorosilicates that settle on the metal surface as an insoluble scale. Ferric ion (Fe3+) promotes the formation of a passive film to inhibit corrosion.
Phosphoric acid slurry in reaction vessels contains around 20-40 mg/lit of free Sulphuric acid to ensure that no unreacted Rock Phosphate remains in solution. The optimum level of free H2SO4 in slurry differs from rock to rock and it has been experienced that an increase in concentration of above 40 mg/lit will enhance corrosion rate by a factor of 4 to 10.
Presence of abrasive silica in Phosphoric acid slurry results in Erosion corrosion to pumps, agitator impellers and piping at a rate that is much higher. This type of attack is in proportion to the relative velocity between the slurry and surface of the equipment. Higher the velocity of rotating equipments, higher is the rate of corrosion and erosion.
  Increase in the temperature of Phosphoric acid slurry accelerates the rate of corrosion. It has been observed that rate of corrosion gets doubled when the temperature is increased from 75 degree Celsius to 80 degree Celsius. Thus it is always important to maintain the temperature of the slurry well below the design limit.

Materials Used in Phosphoric Acid Plant at RCF Ltd Mumbai

• SRL (Rubber Lined Mild Steel) for tanks, pipeline, agitator blades etc.
• FRP Lining is used in corrosive environment to stop leakages from pipelines, vessels etc
• HDPE is used for gypsum slurry transfer lines etc
• Brick lining has been provided in reaction vessels and acid storage tanks to withstand temperature and wear resistance etc
• Rubber lining has also been provided in storage tanks and vessels to withstand wear and corrosion.
• Lead and lead alloys though are very soft in nature; these offer excellent resistance to weak sulphuric acid. The softness of material can be further improved by addition of Antimony. Because of hazardous nature of lead, it has recently been replaced with CPCV.
• Ultra High Molecular Weight Polyethylene (PE) developed as engineering plastic is very commonly used for pumps and pipelines etc due to their advantages over metals. Many of machine components that were earlier made in metal have been replaced by components made in PE. Other types of engineering plastics developed and used in Phosphoric acid industries are Polypropylene, Polyamide, Polyvinyl chloride.

Increase in molecular weight of Polyethylene enhances the technical properties of the material like

• Impact strength
• Wear resistance
• Energy absorption capacity at high stress
• Heat distortion resistance
• Ultimate tensile strength at elevated temperature
• Stress cracking resistance
• Higher corrosion resistance

New Developments of Grinding Media Balls Material
Grinding media balls are required to grind Rock Phosphate in Ball Mill up to the desired size of 200 mesh. These get deformed or damaged during the operation inside the mill.
After carrying out technical survey of literature and post discussions with metallurgists and experienced engineers modifications were made to the composition of grinding media balls that were used in Phosphoric Acid Plant Ball Mill and that is giving good service. This has resulted in drastic reduction in consumption of grinding media balls. Consumption of grinding media balls is approximately 0.2 kg per metric tonne of Rock Phosphate as against the design value of 0.6 kg per metric tonne of Rock Phosphate grinded. Modified composition as per IS 6079 of 1980 is
Low Alloy Cast Steel Grinding Media Balls As Per I S  - 228
Carbon = 1.5 - 2.0 %
Manganese = 0.9 - 1.6 %
Chromium  = 1.6 - 2.4 %
Sulphur = 0.06 % Max.
Phosphorous = 0.06 % Max.
Hardness as per I S  - 1500 of 1968 should not be less than 375 H B at any point across the section of the ball.

Special Alloy Steel Materials Used in Phosphoric Acid Plants
Some special alloy steel materials have been developed and are frequently used in Phosphoric acid services. These materials have been performing satisfactorily resulting in enhanced plant production and efficiency and also avoid unprecedented plant shutdowns.
(a) HV-9 - It is used for acid and slurry pumps and agitators. It is a very costly material .It is used only for critical areas even though its resistance to corrosive atmosphere is very high.
It Contains Cr =  21 - 23%, Ni = 25 - 27%, Mo = 4-6 %, Mn = 2.5%, Si = 1.0%,, P= 0.04%, Ti = 0.25%, C = 0.05%
This material has been further modified in various grades like HV 9A   HV 93, HV 90 A  for better corrosion resistance services and the composition of the above materials are as follows -

 (i) HV- 9A It contains   Carbon - 0.04 % Max. ,Si.= 1.0 % Max., Mn = 2.0 % Max. P = 0.03 % Max. ,S = 0.03 % Max.,Cr. = 20 - 23 % ,Ni = 24 - 26 % ,Mo = 4.5 Cu = 1-2 %, Nb

 (ii) HV- 93 The composition is as follows - Carbon = 0.03 % Max.. Si = 1 % Max,, Mn = 2 % Max., P = 0.03 % Max S = 0.03 % Max. Cr = 19 - 22 ,Ni = 23 - 27 %,Mo = 4.8 - 5.5 ,Nb.

(iii) HV 90 A It contains Carbon = 0.03 % Max ,Si = 1 % Max, Mn = 2 % Max ,P = 0.03 % Max, S = 0.03 % Max., Cr = 19 - 22 % ,Ni = 30 %.Mo = 4.8 - 5.5 , Nb..

  (b) JESSOP-700 - It is  also used for Phosphoric acid slurry pumps etc.
Composition - C = 0.03%, Cr = 21%, Ni = 25%, Mo = 4.5%, Mn = 1.7%,
      Si = 0.5%,Cd = 0.3%

   (c) CD4MCU - I is a cheaper material than alloy-20 or HV-9. It is extensively used for impellers, shafts, sleeves, valves, pump casings and other components.     It is giving  very good service in Phosphoric Acid Plant for various services & use.
Its Composition is as follows - C = 0.04%, Cr = 24.5%, Ni = 4.75 - 6%, Mn = 1.75 - 2.25%, Mn = 1.0%, Si = 1.0%, P = 0,04%, S = 0.04%, Cu = 2.75 - 3.25%, Fe = Balance.

  (d) URENUS - B6 - It is used for seal tank pumps and agitators.
Composition - C = 0.02%, Cr = 19 - 22%, Ni = 24 - 27%, Mo = 4 - 4.8%, Mn = 2%, Si = 0.5%, P = 0.04%, Cu = 1 - 2%, S = 0.015%

   (e) SANICRO - 28 - It is very good material but it is very costly material and   can be used extensively  for mixing head of Sulphuric al cid and weak phosphoric acid. along  with Phosphoric Acid  Plant other services.
Composition - C = 0.03%, Cr = 26-28%, Ni = 30 -34%, Mo = 3.04 - 4%, Mn = 2.5%, Si = 1%, P = 0.03%, S = 0.03%, Cu = 0.6 - 1.4%

    (f) UDELHOM-U904L -It is used for gypsum belt filter vacuum box.
Composition - C = 0.02%, Cu = 1..2- 2%, Mn = 2%, Cr = 19 - 21%, Ni = 24.5 - 26%, Mo = 4.5-5%, Si = 1.0%, P = 0.03%, S= 0.015%

    (g) CARPENTER - 20 - It is also used for Phosphoric acid pump impellors etc.
Composition - C = 0.07%, Cd = 0.56%, S= 1.0%, Cu = 3 - 4%, Si = 1.0%, Mn = 2%, Mo = 2 - 3%, Ni = 32 - 38%, Cr = 19 - 21%

   (h) CA-A-700 -  It is used for phosphoric acid transfer pumps etc.
Composition - C= 0.1%, Cr = 22 - 25%, Ni = 19 - 22%, Mo = 1.75 - 2.75%, Mn = 2%, si = 1.5%

    (i ) DURIMATE-20 - It is also used for Phosphoric acid transfer pumps etc.
Composition - C = 0.07%, Cr = 20%, Ni = 29%, Mo = 2%, Si = 1.0%, Cu = 3.0%
(j) DURIMATE-100 - It is a  substitute of CD4MCU is used for all the services of Phosphoric Acid  and where C D 4 M C U is

(k) AL- BRONZE - It is used for bearing housing of phosphoric acid pumps.
Composition - Cu = 82 - 86%, Al = 12%,Fe = 3.06%, Zn = 0.15%, Ni = 0.19%

(l) CARBON&GRAPHITE-It has good resistance to Phosphoric Acid,,Heat Exchanger Tubes, Pumps, Equipment parts after impregnating them with Phenol ic furnace or Epoxy resins.

Conclusion
Major problems associated with industrial corrosion can be taken care of by the design engineers by proper selection of materials of construction for various process equipments and structures.
With the availability of improved metals and alloys, life of most of equipments can be calculated. Modern technology and superior materials of construction have extended the life span of process equipments significantly.
Metallurgists and Engineers have been working diligently towards developing new corrosion resistant materials to avoid premature failures of plant and machinery to overcome plant shut downs eventually prevent production losses. Almost every year newer materials are introduced for commercial use in the industry for improved safety and also huge cost savings.

RP Sharma is Deputy General Manager with
RCF Ltd Mumbai.
E mail:  rpsharma@rcfltd.com