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.
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.
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.
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.
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)
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.
Failure mechanism affecting aluminium in Gas Plant (reaction with Hg) There are three mechanism that Hg attack to aluminum
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)
LME process AL 2 O 3 Aluminum Liquid Hg (-38,9C) Crack For crack propagation stress are required
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).
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
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”)
Many commonly used materials, low alloy steel, high strength steel, 300 series SS, nickel based alloys, copper alloys, aluminum and titanium alloys .
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 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
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.