Aluminium Corrosion Alodine Protective Coating

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  • 1. Alodine EC2-Protective Coating from Mercury attack to aluminum and titanium alloys in Oil&Gas equipment By Fernando Vicente Reliability and Integrity Engineer
  • 2.
    • With an annual world consumption of 25 million tons, aluminum is the leader in the metallurgy of non-ferrous metals.
    • The development of applications for aluminums and its alloys, as well as the sustained rise consumption can be attributed to several of its properties which are decisive in user’s choice of metals.
    • Properties:
    • Lightness
    • Thermal conductivity
    • Electrical conductivity
    • Suitability for surface treatments
    • In most of gas plant (LNG,NGL and regasification) typical equipments and its parts made of aluminum or its alloys are: Heat exchanger tubes, turbo expander impellers and cages, valves, piping, some internal part of pressure vessel.
    The Advantage of aluminum
  • 3.
    • Mercury is present either as a metal in vapor phase or as an organometallic compound liquid fractions. Concentration levels are generally very small (less than 100  g/Nm3), but even at very small concentration levels, mercury can be detrimental due to its toxicity and its corrosive properties when reacts with aluminum alloys.
    • Special materials are often required in a gas processing plant construction phase for most important equipments (compressors, heat exchanger tubes, Cold Boxes, valves, piping etc) due to components such as hydrogen sulfide, carbon dioxide , mercury and water. The availability and cost of these materials and their delivery should be revised. Sometimes cladding or linings may be alternatives to expensive and scarce alloys. In addition, approved welding procedures may not be available or the work force could not be an expertise for special alloys, increasing wasting time and costs.
    • This presentation intends to show a “potential” solution to avoid aluminum corrosion in gas processing plant, reducing maintenance cost and time due to future inspection, reducing the risk of catastrophic failure.
    Introduction
  • 4. TYPICAL MERCURY CONCENTRATION IN NATURAL GAS LOCATION (μg/Nm3) Groningen (Hol.) (gas well) 180 - 200 Arun (Indonesia) (gas well) 250 – 300 Germany North (gas well) 15 – 450 Germany South (gas well) <0,1 - 0,3 EE.UU. east (pipeline) 0,02 – 0,4 Algeria (pipeline) 0,1 – 89 Skikda (Algeria) (plant) 0,001 – 0,65 Wyoming (EE.UU.) (plant) 8 – 24 Venezuela (plant) 0,6 – 1,3
  • 5.
    • Like copper salts, mercury salts lead to sever pitting corrosion of aluminum. Due to its volatility, mercury can easily be transported by moving fluid such as natural gas that contains minute traces. Experience shows that mercury may be concentrated in plants where natural gas undergoes liquefaction and regasification, (LNG,NGL and regasification plants). It may damage heat exchangers, turbo expander impellers, valves and Cold Boxes.
    • Mercury itself leads to sever corrosion of aluminum, which appears as a very narrow white lines, possibly thicker than 1cm. It may also lead to intercrystaline corrosion and rupture at cracks
    • Crack in pressure vessel, Cold Box or Heat exchanger subject to high pressure, temperature and containing explosive or toxic fluid could be an undesirable scenario for a gas plant and community. Furthermore, this type of equipment are very difficult to inspect in-service and out of service as well , due to geometry complexity.
    • For this reason a good Reliability Engineering practice to avoid high risk scenarios and maintenance cost is to apply a protective coating in those equipment under mercury attack during plant design phase.
    Hg the primary enemy of Al in gas stream
  • 6.
    • The mechanism of attack of aluminum by mercury is rather complex. A spontaneous reaction between mercury film, aluminum, humidity and oxygen from air occurs. While mercury is insoluble in aluminum, aluminum is slightly soluble in mercury (0,002% at room temperature). When mercury is wetting the aluminum surface, it keeps it activated, because no oxide layer can form. Aluminum will dissolve in the mercury and become oxidized in contact with air.
    • Eventually, the liquid mercury will transport aluminum to the outside, where it becomes oxidized in contact with air and humidity. There is no consumption of mercury during this reaction, which, once started , will never stop.
    • This process is a typical Galvanic Corrosion due to difference in mV from one metal to other (Al-Hg).
    • A typical solution to avoid this corrosion process is to apply current or apply a specific coating between materials (Al-Hg)
    Aluminum corrosion-General
  • 7.  
  • 8.
    • The equipment failure made of aluminum in process gas plant or ethylene plant caused by Hg in liquid state has been studied since 19763 due to a heat exchanger failure in Algeria LNG plant.
    • Related failures:
    • Moomba, Australia, LNG plant (01/01/04)
    • Skikda, Algeria,LNG plant (01/19/04)
    Impact in aluminum alloys Catastrophic failure in Skikda plant Source: “ The 50 Major Engineering Failures “(1977-2007)
  • 9.
    • Mercury amalgamates with aluminum with difficulty because the natural oxide film on aluminum prevents metal-to metal contact. However, after the two metals have been in contact, due to oxide film is broken, amalgamation occurs immediately, and in the presence of moisture, corrosion of aluminum proceeds rapidly. The effects can be sever when stress is present. For example, attack by mercury and zinc amalgam combined with residual stress from welding manufacturing process cause cracking of weldment. The corrosive action of mercury can be serious with or without stress because amalgamation, once initiated, continues to propagate unless the mercury can be removed.
  • 10. Failure mechanism affecting aluminium in Gas Plant (reaction with Hg) There are three mechanism that Hg attack to aluminum
  • 11.
    • Is the process by which Hg forms liquid solution with metals like, Al, Sn, Au, Ag y Zn , no water is required to be developed.
    • In case of formation with Al, this process face two problems:
    • The aluminum oxidized film offers a relative protection, however can be present some failure in different points (depend of the manufacture)
    • The Al solubility in amalgam is low, and require high quantities of HG to dissolve a low Al quantities. It manifest like a “Pitting Corrosion”
    • “ The pure amalgamation has not been observed in field failures”
    Amalgam Hg AL
  • 12. Amalgam corrosion
    • Occurs when Hg and Al create amalgam in humidity presence:
    • Hg + Al  Hg(Al) (amalgamation)
    • Hg(Al) + 6 H2O  Al2O3.3H2O + 3 H2 + Hg
    • Hg + Al  Hg(Al) (amalgamation)
    Due to Hg regenerate itself, the reaction is self propagating while water exist . The amalgam corrosion is accelerate in O 2 presence, and creates a filiform corrosion (Al 2 O 3 feather)
  • 13.
    • Al 2 O 3 feather in aluminum prove that has been immersed in liquid Hg
    Amalgam corrosion
  • 14.
    • The difference between simple amalgamation and amalgam corrosion is that the last requires water to propagate and propagates with very small Hg quantities.
    • This condition can be present during equipment opening for maintenance inspections and plant shutdowns.
    • If there are a high humidity and Hg enough quantities, the process could be accelerated in a significant way but not as much as LME.
    • Amalgam corrosion it is not a common failure mechanism because of there are no contact between Hg (liquid) and humidity or water in oil and gas process (cryogenic and regasification).
    • Occasionally, pitting with white colors can be easy found with boroscope
    Amalgam corrosion
  • 15.
    • This is the most important of three failure mechanism that attack to aluminum in gas plants.
    • Amalgamation occurs in grain boundary followed by an intergranular stress cracking activated by stress and or residual stress.
    • No water is required, however the water presence get worse the scenario.
    • In case of aluminum alloys this attack has been observed where magnesium precipitates in grain boundary like Al 3 Mg 2 during equipment manufacturing process or welding process. However, some events are required to be effective the LME attack:
    • The Hg needs to be in liquid phase (i.e -38,9ºC)
    • Exist a crack in the aluminum protective oxidized film
    LME (Liquified Metal Embrittlement)
  • 16. LME process AL 2 O 3 Aluminum Liquid Hg (-38,9C) Crack For crack propagation stress are required
  • 17. Alloys composition susceptible to aluminum LME attack (% p/p) ALLOY Cr Cu Mg Others 5083 0,05 -0,25 < 0,1 4,0 – 4,9 Mn 0,4 – 1,0 5086 0,05- 0,25 <0,1 3,5 – 4,5 Mn 0,2 – 0,7 6061 0,04 -0,35 0,15 -0,40 0,8 – 1,2 Si 0,4 – 0,8 7075 0,18 -0,28 1,2 – 2,0 2,1 – 2,9 Zn 5,1 - 6,1
  • 18.
    • The cryogenic heat exchanger failures induced by LME mechanism has been identified in piping heading made of aluminum alloys that contains magnesium, most of this has been found in welding of aluminum alloys 5083,5086, and 6061.
    • Cracks has been detected after 7 or 8 years in service. Most cracks has been observed in heading corners and circumferential piping weld seam (where us the maximum hoop stress).
    LME failure history
  • 19. 5083 ó 5086 Alloys Alloy 3003 ( Cold box )
  • 20. LME (Crack in piping heading)
  • 21. LME (Crack in circumferential weld seam)
  • 22.  
  • 23.
    • Furthermore, some failures caused by Stress Corrosion Cracking has been reported in turbo expanders and JT Valves (Joule Thompson) from cryogenic trains in NGL and LNG plants, made of in aluminum alloys 6061 and 7075.
    • “ Turbo expander impeller failure,” alloy 6061.
    LME failure history Source:B. Bavarian, CORROSION 2004 (NACE), paper 558
  • 24. LME cracks
  • 25. LME cracks in rotor, alloy 7075
  • 26.
    • Description of damage
    • Liquid Metal Embrittlement (LME) is a form of cracking that results when certain molten materials come in contact with specific alloys. Cracking can be sudden and brittle nature. (“this is a very key issue for integrity gas plant”)
    • Affected material
    • Many commonly used materials, low alloy steel, high strength steel, 300 series SS, nickel based alloys, copper alloys, aluminum and titanium alloys .
    • Critical Factors
    • LME occurs in very specific combinations of metal in contact with low melting point materials such as zinc, mercury, cadmium, lead, copper and tin.
    • High tensile stress promote cracking, however cracking can initiate simple through contacting the molten metal with susceptible alloy. Very small quantities of low melting point metal (Hg) are sufficient to cause LME.
    • Tensile stress contribute to crack propagation rates. Cracking under load can be extremely rapid such that cracks may pass through the wall within seconds of contact with molten metal.
    • Cracking can occur after long periods of time when contaminated surface are expose to liquid metal.
    LME
  • 27. LME
    • Affected units or equipments
    • LME can occur in any location where the LME couples are found. In refineries, mercury is found in some crude oils and can be condense in the atmospheric tower overhead system thereby embrittling brass, Alloy 400, titanium or aluminum heat exchanger components.
    • Failure process of instruments that utilize mercury can introduce the liquid metal into refinery streams
    • LME of aluminum components has occurred in cryogenic gas plant components due to condensation of liquid mercury
    • Appearance or morphology of damage
    • Damage resulting from LME will appear as brittle cracks in an otherwise in ductile material.LME can only be confirmed through metallographic by the presence of intergranular cracks, usually filled with low melting metal.
    • Techniques such as spectrographic analysis may be required to confirm the presence of the molten species.
    • Prevention and mitigation
    • LME can only be preventive by protecting the surface metal with and specific protective coating
  • 28. LME
  • 29.
    • For cryogenic gas plant, equipment made of aluminum, the mercury concentration levels for feed gas follows this suggestions:
    • Mercury concentration < 0,01  g/Nm 3 : Tolerable (no action is required)
    • Mercury concentration >0,01  g/Nm 3 < 0,1  g/Nm 3 : Tolerable, if equipment are good designed, this means mercury resistance. Protective coating is required.
    • Mercury concentration >0,1  g/Nm 3 : Mercury removal and protective coating is required.
    Mercury levels for cryogenic gas plant
  • 30. Typical cryogenic plant Affected equipment
  • 31.
    • Temperature: from 100C to -98C
    • Pressure: 30Bar to 70 bar
    Typical process parameters
  • 32.
    • Decrease maintenance cost (no plant shutdown is required to inspect amalgamation)
    • Increase equipment reliability parameters (MTBF-Mean Time Between Failures)
    • Increase mechanical integrity state
    • Reduce unplanned plant shutdown due to equipment failures
    • Reduce the risk scenario from a catastrophic failure
    • Reduce spare part inventory
    • Increase the gas plant safety
    Benefit from protective coating
  • 33.
    • The mercury transported by the natural gas has produced equipment failures in cryogenic plants. Most of this failures had been reported in equipment made of aluminum alloys from 5000,6000 and 7000 series. Typical equipment from cryogenic trains are, cold box, piping, JT valves and turbo expander.
    • For those equipment subject to mercury attack should be necessary a redesign review to increase reliability values.
    • Special protective coating may be applied to those equipment during manufacture process to increase the service life in gas plant.
    • Reducing maintenance cost ,maximizing the equipment integrity and increase the safety of the gas plant.
    Summary
  • 34.
    • API RP- 571 : Damage Mechanism Affecting Fixed Equipment in Refining Indsutry , 1 st edn, Washington, D.C, December 2003
    • API PUBL.581 : Risk Based Inspection Based Resource Document , 1 st edn, Washington, D.C, May 2000
    • JMC Campbell ,Technical Assistance Service for Design, Operation and Maintenace of Gas Plants , Washington, D.C, September 2003.
    • ASM Metals Handbook Volumen 13, Corrosion, Printed in United State of America in 1992
    •   Christian Vargel, Corrosion of Aluminum , Elseiver, March 2004
    Bibliography