Effect of Mercury on Engineering Materials Used in Ammonia Plants
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Effect of Mercury on Engineering Materials Used in Ammonia Plants Inappropriate combinations of moisture, alloy and mercury can accelerate corrosion and form explosive compounds. Are there ways of preventing these potential hazards? S.Mark Wilhelm Cortest Laboratories, Inc., Cypress, TX 77429 INTRODUCTION Secondly, concentration of mercury in LNG and ethylene plants has Elemental mercury (Hg) is becoming presented a range of materials an increasingly more prevalent related problems having to do with contaminant to natural gas feed aluminum. Lastly, reaction of stocks due to its production from mercury with ammonia (or NH3 various reservoirs around the world. precursors) can deposit potentially Table 1 shows the origin of gas explosive compounds in NH3 process having mercury contamination from a equipment. variety of locations(1>. This paper discusses the following In general, mercury contamination of points relating to mercury and its gas streams does not cause major influence on gas processing and use: operational problems for gas production, transportation or 1. Origin and composition of processing, at least from the mercury and mercury compounds standpoint of engineering. There in natural gas. are, however, environmental concerns from end use"'. Environmental issues Materials degradation will not be addressed in the present mechanisms. discussion. 3. Overview of mercury related Several situations have come to equipment failures. notice since 1975 that cause concern for the gas industry as they relate 4. Mercury - nitrogen compounds. to mercury contaminated gas. One such problem is that minuscule 5. Recommendations for ammonia amounts of mercury in gas can induce plant operations. liquid metal cracking of a small group of susceptible alloys. The Mercury in Natural Gas problem has been manifested in the brittle failure of values and Table 1, as mentioned, provides the specialty components in production concentrations of mercury found in equipment. natural gas produced from selected 20 locations'1'. Mercury is scarce in 2. amalgam corrosion domestic (USA) supplies but has been detected in sizable proportions in 3. liquid metal embrittlement gas from North Germany, Algeria and Indonesia. The predominant form of 4. galvanic corrosion mercury is elemental; organic complexes account for less than 2 Amalgamation percent of produced mercury. Amalgamation is the process by which Sampling and detection of mercury is mercury forms liquid solutions with an inexact science and is various metals, primarily aluminum, considerably difficult with tin, gold, silver and zinc. Of inexperienced personnel. There can these, only aluminum has mechanical be, therefore, large uncertainties significance. The affinity of in measured concentrations aluminum for mercury is mediated by especially in liquid samples. The the oxide (A12O3) that protects the theoretical maximum amount of aluminum surface. Figure 2 mercury in gas is estimated by its illustrates the typical situation in vapor pressure at the temperature of which mercury contacts the aluminum the reservoir (see Figure 1). In all oxide surface layer protecting the known situations, the actual underlying metal. The oxide on measured concentration is less (by aluminum is not homogenous and at least an order of magnitude) than contains numerous defects. In the theoretical maximum deprived general, mercury lacks the ability; from the vapor pressure. This fact because of surface tension, to suggests that liquid mercury is not diffuse through cracks or defects a coexisting phase in gas reservoirs and hence cannot reach the and offers some reassurance that underlying metal. This lack of elemental mercury particulates will wettability, however, can be not be present in produced gas. mitigated by thermal or mechanical stresses, by chemical environments Mercury does not form stable and by temperature. complexes with most hydrocarbon derivatives and is not expected, nor In situations in which Hg can breach found, to exist as an organometallic the aluminum oxide and wet the species in any significant underlying surface, the rate of quantities. amalgamation depends in a major way on metallurgical (microstructural) Mercury adsorbs readily on condition. It is observed, for production and transmission instance, that mercury amalgamates equipment surfaces and, therefore, selectively with weldments and more its gas phase concentration is rapidly with particular alloys. location dependent, dropping off Table 2<1) provides a list of with distance from the wellhead. aluminum alloys and the kinetic Similarly, mercury has a significant degree to which they amalgamate with affinity for liquid hydrocarbon and Hg. partitions to the liquid phase in condensate separators, sweetening The primary manifestation of equipment, etc. The concentration of amalgamation is loss of mechanical mercury in coexisting hydrocarbon strength in weldments. Amalgamation liquid/methane can be estimated as does not require stress nor does it shown in Figure 1. require mediation by a conducting electrolyte to occur. MATERIALS DEGRADATION MECHANISMS Amalgam Corrosion Mercury can degrade materials by four basic mechanisms: Amalgam corrosion is the combined action of Hg and moisture on 1. amalgamation susceptible materials, primarily aluminum and tin. The difference 21 between this mode of attack and surface contamination. The limiting simple amalgamation is that the crack velocity is approximately 100 corrosion process propagates with cm/s which is reached due to the minuscule amounts of mercury. The limit of liquid diffusion of mercury reaction scheme is: (not surface diffusion or vapor diffusion). In Figure 4, the Hg + Al Hg(Al) amalgam limiting stress intensity factor for [1] mercury LME of aluminum is shown. For weldments it can approach 5 ksi Hg(Al) + 6 H2O 3H2 in1/z which translates into a Hg [2] critical defect size of 10~* mm. LME is not limited to aluminum but Hg + Al -» Hg(Al) [1] can affect high strength steels as well. Susceptible materials include etc. 4140 RC 35-40 at elevated temperature and precipitation Amalgam corrosion regenerates the hardened stainless steels also reactant and hence is self- having high strength and high propagating. If sufficient moisture hardness. and mercury are present, aluminum structural components can be Galvanic Corrosion penetrated fairly rapidly. The rate of attack is mass transfer limited Galvanic corrosion caused by mercury but does not proceed as rapidly as deposits in normally corrosive liquid metal embrittlement (LME) environments has been observed for discussed below. Amalgam corrosion steels. Mercury serves to accelerate is selective to the same alloys as acid dissolution and to aggravate simple amalgamation, however, it can localized corrosion such as pitting. affect all aluminum alloys to some The situations in which mercury degree. deposits contact steel are found in gas/water separators in gas Liquid Metal Embrittlement (LME) production and when mercury from gauges contaminates process piping Liquid metal embrittlement by in chemical plants. mercury is distinct from amalgamation in that it produces MERCURY RELATED FAILURES IN GAS rapid brittle fracture and affects a PROCESSING EQUIPMENT much broader range of materials (aluminum, nickel-copper alloys, LME of Aluminum Cold Box Pipina(5) brasses, copper alloys, tin alloys, some stainless steels). The Ethylene plant cold box piping (see mechanism of LME involves liquid Figure 5) has experienced LME due to diffusion of mercury atoms in grain contact of elemental mercury with boundaries. Cracks usually initiate aluminum alloy 5083 weldments. The and propagate via the grain mercury originated in Algerian boundaries. LME is distinct from feedstocks (LNG) that had been stress corrosion cracking in that no processed through the cold box for a purely electrochemical processes are considerable period of time. The involved. mercury contamination was detected ultrasonically and radiographically. The unusual aspects of LME, as Failure occurred upon start-up after opposed to other fracture processes, a plant shutdown at weldments in are that the crack propagation rate cold box piping. can be exceedingly fast and the stress intensity required for crack Many ethylene plants have now propagation can be very low. Figure installed sulfur-imprégnated carbon 3 illustrates the rapidity with filters to adsorb mercury in inlet which aluminum cracks propagate in gas. Methods to clean existing smooth tensile specimens having equipment that may be contaminated 22 (1) have not been developed as yet. Corrosion of Stainless Steel Detection and location of cracks is very difficult. Acoustic emission 316 L stainless was severely shows some promise, but accurate corroded when subjected to a mixture procedures to measure existing of CH4, CO2, H2S water and mercury. damage are still in the Both water and mercury were developmental stage. required. Corrosion took the form of pitting and crevice attack. The LME of Monel 400 Valve Stems and affected equipment was a laboratory Springs research pressure vessel. Valve springs and valve stems of CHEMICAL REACTIONS OF MERCURY IN Monel 400 were found to experience AMMONIA SYNTHESIS LME upon exposure to raw natural gas containing approximately 100 ug/m3 Mercury - nitrogen compounds are elemental mercury. No condensing notoriously explosive and include: mechanisms (i.e. cold temperatures) were present. Mercury was delivered mercury nitride to the susceptible pieces by Hg2N3 adsorption from the gas and/or condensate. mercury (halide)