Simulation of Electrochemical Processes 59 Galvanic corrosion modeling in flowing systems D. A. Shifler 102 Potomac Street, Boonsboro, MD 21713-1211, USA Abstract Contact between dissimilar alloys will generate galvanic currents based on a variety of factors. The extent of risk to the overall systems and specifically to the more electronegative alloy materials can be clarified if the magnitude of the corrosion rate at various locations can be modeled and calculated. When a more electropositive cathode alloy is separated from the electronegative anode alloy in a complex geometry, there will be a variable distance between the anode and cathode and fluid flow characteristics may influence variations in the limiting current density. In systems containing elbows, the current and potential distribution will not be only in the longitudinal direction, but will have a radial contribution to current flow and corresponding potential changes. Keywords: galvanic corrosion, current distribution, flow, modeling, piping, boundary layer, polarization, dissimilar alloys, bimetallic corrosion. 1 Introduction The effect of coupling two different metals/alloys together, either directly or through an external path, increases the corrosion rate of the anodic alloy (the material with the more electronegative potential) and reduces or suppresses the corrosion rate the cathodic alloy (the material possessing the more electropositive potential). The electrochemical degradation derived from the joining of two or more different or dissimilar alloys is termed galvanic corrosion. Galvanic corrosion causes failures in a wide variety of environments such as atmospheric, automotive, seawater, soil, freshwater, or oil and gas. The necessary conditions for galvanic corrosion require: (1) dissimilar alloys, (2) electrical contact, either directly or by a secondary, grounding path, between the dissimilar alloys, and (3) an electrolyte. The coupling of dissimilar alloys in conducting, corrosive solutions such as seawater, oil and gas, and biological
60 Simulation of Electrochemical Processes fluids can lead to accelerated corrosion of the more anodic, electronegative alloy and protection of the more cathodic, electropositive alloy. The extent of galvanic corrosion depends on factors such as: (1) the type of joint; (2) the effective area ratio of the anodic and cathodic members of the galvanic couple; (3) geometry of the coupling members; (4) mass transport (convection, migration, and/or diffusion); (5) bulk solution properties (oxygen content, ph, conductivity, pollutant/contamination level); (6) temperature; (7) flow rate; (8) volume of solution; (9) major and minor alloy constituents; (10) protective film characteristics (oxide, salt films, etc.) in the given environment; (11) surface condition; (12) reaction kinetics (metal dissolution of the anodic member, and either oxygen reduction or hydrogen evolution overvoltages, or the cathodic efficiency of the more noble, cathodic alloy); and (13) the difference in potential between the alloys [1, 2]. Figure 1 shows these and other factors that affect galvanic corrosion [1]. Figure 1: Factors affecting galvanic corrosion [3]. Usually galvanic couples involve electrolytes containing dissolved oxygen. When the galvanic current is limited by oxygen diffusion to the cathode, it is under cathodic control. Under this condition the galvanic current rate is directly proportional to the cathode area and independent of the anode area [3]. Later referred to as the catchment area principle, it also illustrates that the anodic current density is inversely proportional to the anode area. Therefore, the area of the cathode should be minimized so that the cathode/anode area ratio is very small for better long-term performance. The composition of the electrolyte influences the magnitude, the distribution, and often the direction of the galvanic current flow. High-conductivity fluids tend to spread out the galvanic currents over larger surface areas, when possible, while low conductivity fluids will limit the galvanic current distribution to just the cathode/anode junction [4].
Simulation of Electrochemical Processes 61 Galvanic corrosion is sensitive to soluble ions in the electrolyte that affect the stability of passive films on the cathodic member of the galvanic couple or the kinetics of the corroding anodic member. The presence of anions such as bicarbonate, silicate, and sulfate may promote the development of insoluble, adherent corrosion products that protect the anodic member from further corrosion and change the relative electrochemical potentials of the galvanic couple based on the relative oxygen concentrations at the surface of each alloy. Other organic or inorganic complexing ions may accelerate corrosion of particular metals and alloys. The magnitude of the potential difference between the dissimilar materials does not indicate the degree of galvanic corrosion because potentials are a function of the thermodynamics and not of the reaction kinetics that may occur. A difference of 50 mv between select dissimilar materials can lead to severe corrosion while a potential difference of 800 mv has been successfully coupled [5]. Temperature influences the magnitude and direction of galvanic corrosion current. For a zinc/steel or aluminum/steel couple, increasing temperature above 60 C or more may result in polarity reversal due to a change primarily in the zinc potential aided by the presence of bicarbonates and nitrates in the water [6,7]. When dissimilar alloys are coupled, the rate of dissolution at the anode or the reduction reaction rate at the cathode is not equal for all metals/alloys. The reaction rate at the anode or cathode surface determines the electrode efficiency. Polarization caused by current flow between the dissimilar metals/alloys causes a shift in the electrochemical potential of the anodic alloy to a more electropositive value and a shift of the cathodic member to a more electronegative value. The extent of polarization will depend on the metal/alloy and the specific environment to which the galvanic couple is exposed. Generally, in neutral environments, the galvanic couple will be under cathodic control. The extent of cathodic polarization will determine how well a material will drive the corrosion of the anode. Highly polarizable cathodes (alloys with low cathodic efficiency) will tend to cause relatively benign galvanic corrosion, while high cathodic efficient materials can promote severe galvanic corrosion. However, cathodic efficiency can change under different conditions that affect the material surface kinetics [5]. Galvanic relationships between different alloys may be altered in sulfide-polluted seawater [9] because of changes in the stability of the passive films. Using computer models to predict the magnitude of galvanic corrosion or the current demand for cathodic protection systems in seawater or other electrolytes requires accurate polarization data for the materials involved [9]. However, polarization curves for most structural materials are exposure-time and scan-rate dependent, thus complicating long-term galvanic corrosion predictions [9]. Platinum, silver, and copper form naturally thin formed oxide films and are easily reduced to metal; these alloys are efficient cathodes and, therefore, tend to promote galvanic corrosion [5]. The increasing use of nonmetallic materials (metal-reinforced composites, graphite metal matrix composites, conducing polymers, semi-conducting metal oxides, and conducting inorganic compounds)
62 Simulation of Electrochemical Processes often exhibit potentials that are more electropositive than most alloys, and when coupled to most alloys, cause galvanic corrosion [10]. In contrast, aluminum, stainless steels, and titanium all have a stable oxide film and tend to polarize and hence, are poor cathodes. In flowing, aerated seawater the oxide film is likely to thicken, thus diminishing galvanic corrosion of the coupled metal further. 2 Fluid flow Fluid movement, usually characterized by velocity or dimensionless Reynolds number, can have an extreme effect on the corrosion behavior of metals and alloys in fluids. An increase in fluid flow may increase corrosion rates by removing protective films or by increasing the diffusion or migration of deleterious species. Conversely, increased flow may decrease corrosion rates by eliminating aggressive ion concentration or enhancing passivation or inhibition by transporting the protective species to the fluid/metal interface. Thus flow, in combination of other factors, may strongly influence galvanic corrosion. In any given velocity domain, local velocities permutations may exist over diverse areas of a component due to factors such as geometry, flow disruptions, or mode of fabrication [11]. Turbulent flow increases agitation of waters with the structural materials more than in laminar flow, with the resultant probability of increased galvanic corrosion. Flow above a critical velocity in some alloys such as copper-based alloys are susceptible to a critical surface shear stress in seawater where the corrosion product film begins to breakdown and is removed from the alloy surface, subsequently causing accelerated attack [12]. Titanium and some nickel-chromium-molybdenum alloys perform well in low, intermediate, and high velocity ranges up to 120 fps (36 m/s). 3 Galvanic corrosion: effects by the environment The effective galvanic length in piping systems is the distance that galvanic current flows due to the presence of a dissimilar metal couple and may range from 15 to over 200 pipe diameters [13-16]. Steel coupled to stainless steels and copper-nickel will have the tendency to form calcareous deposits on the cathodic members of the couple, thereby reducing galvanic currents. The galvanic corrosion rate of many copper and nickel-based alloys, and of stainless steels in seawater, depends upon the flow rate as well as the area ratio. Copper and copper-nickel alloys tend to become more noble, i.e., more electropositive, and corrode less as the flow rate increases. In well-aerated, flowing solutions nickel-based alloys and stainless steels are also likely to become more passive and corrode less than under stagnant conditions [5]. In general, the rate of corrosion of coupled and non-coupled metals decreases with exposure time. This is generally due to the diminishing rate of diffusion of oxygen (or hydrogen) through the corrosion product films at cathodic regions and corresponding protection afforded by the corrosion product at anodic sites.
Simulation of Electrochemical Processes 63 Galvanic Currents Current (ua) 7000 6000 5000 4000 3000 2000 1000 0 0 200 400 600 800 Time ( Days) Loop 1 Loop 7 Figure 2: Titanium/70:30 Copper-nickel piping couples-measured galvanic currents per exposure time. Current (ua) 8000 7000 6000 5000 4000 3000 2000 1000 0 0 100 200 300 400 Time (days) Loop 6 Loop 12 Figure 3: Alloy 625/70:30 Copper-nickel piping couples-measured galvanic currents as a function of exposure time. Research was done to observe the galvanic corrosion effects of coupling straight titanium alloy or Alloy 625 piping sections to straight 70:30 coppernickel piping in flowing (at 6 fps (1.8 m/s)) seawater [17]. Both titanium and Alloy 625 pipes had similar cathodic potentials and generated similar galvanic currents when coupled to copper nickel piping. Both uncoated titanium and Alloy 625 piping sections experienced ennoblement of several hundred millivolts between 14 and 65 days. The galvanic currents increased 7-14 days after ennoblement occurred as shown in Figures 2 and 3. Ennoblement of the cathode (titanium in this case) increased the potential difference between the anode and the cathode as shown in Figure 4. The phenomenon of ennoblement is likely caused by microbial action that increases the cathodic kinetics, which shift the cathodic polarization curves to higher currents, thus increasing galvanic currents on the anode [18-20]. It is also observed during the 600+ day exposure that the galvanic currents and the potentials of both the cathode and the anode varied. The cathodic potential and the resulting galvanic current tended to decrease with
64 Simulation of Electrochemical Processes longer exposure times due to the generation of thicker films and resultant lower cathodic efficiency. Potential (mv vs Ag/agCl) 300 100-100 -300 Potential Profile - Loop 1 0 5 10 15 20 2 days 14 days 100 days 190 days 247 days 331 days 451 days 546 days 637 days Distance from Inlet (ft.) Figure 4: Titanium/70:30 Copper-nickel piping couple-potential profile of Loop No. 1 as a function of time. 4 Galvanic corrosion: modeling and engineering Contact between dissimilar alloys will generate galvanic currents based on factors already delineated. The extent of risk to the overall systems and specifically to more electronegative alloy materials can be clarified if the magnitude of the corrosion rate at various locations can be modeled and calculated. The potential distribution within a cylindrical system at a give time involves solution of the Laplace equation for the electrostatic potential in the seawater at any position given by cylindrical coordinates x (length along cylinder), and r (cylinder radius) [21]. Use of the Laplace equation assumes no concentration gradients in the seawater [22] and that all mass transfer results from ionic migration. Solution of the Laplace partial differential equation is non-trivial even for simple geometries and an analytical solution is not generally possible [21]. Fourier series solutions may be used for simple systems while finite element, finite difference, or boundary element analysis [21]. An alternative approach for simple geometry, cylindrical systems assumes that potential changes only in the longitudinal direction and that there is no radial component contributing to potential changes and that there is linear polarization kinetics [21]. An example of a symmetrical geometry for a dissimilar piping couple that is under cathodic protection in flowing seawater is shown in Figure 4. There the potential distribution is very nearly symmetrical with the asymmetry due the differences in open circuit potential of the Alloy A and the Alloy B pipes, respectively. However, there are a number of examples where system/component geometry involving dissimilar alloys is not symmetrical or simple and the system may involve more than two dissimilar alloys. In addition, the contribution of
Simulation of Electrochemical Processes 65 flow is generally not considered. In systems containing elbows, the current and potential distribution will not be only in the longitudinal direction, but will have a radial contribution to current flow and corresponding potential changes. Since oxygen reduction reaction is the predominate cathodic reaction, the rate of reaction may be limited by oxygen diffusion as described by: i c = 4FD 0 C oxygen /δ where i c is the cathodic current density, F is Faraday s constant, D 0 is the diffusion coefficient in the electrolyte (seawater in this case), C oxygen is the oxygen concentration in the bulk electrolyte, and δ is the thickness of the diffusion layer. The total ohmic potential drop in the electrolyte between any two points on the surface of the anode and the cathode can be calculated. The ohmic potential drop, V d, due to the distance between the anode and cathode is: V d = I a R d = I a R d where R d = ρd/δl with ρ the resistivity of the electrolyte, δ is the electrolyte thickness, d is the distance between the anode and cathode, and l is the width of the electrodes [10]. In the example where the more electropositive cathode is separated from the electronegative anode alloy by an elbow, there will be a variable distance between the anode and cathode leading to an asymmetrical current distribution. The distance between the anode and cathode along the elbow intrados, d i, will be d i = ar i, where a is a constant and r i is the inner elbow radius. The cathode-to-anode distance, d o, along the elbow extrados will be d o = ar o a(r i + z) where r o is the outside radius and z is the inside diameter of the elbow. As the diameter of the elbow increases, the disparity of anode-to-cathode distance along the inner radius verses anode-to-cathode distance along the outer elbow radius increases. Under a thin layer electrolyte, the potential variation is large between the anode and the cathode. However, both the anode and cathode are only slightly polarized, except for the boundary between the cathode and the anode. With decreased fluid velocity, the electrolyte boundary thickness increases, and the galvanic current increases due to lower ohmic resistance. The current distribution become more non-uniform, and in the case where the anode and cathode are separated by an elbow, the galvanic current becomes concentrated along in the inner elbow radius plane since the current will follow a path of least ohmic resistance. Conversely, as the fluid velocity increases, the electrolyte thickness along the piping surfaces becomes thinner, increasing ohmic resistance resulting in a more uniform current distribution around the entire anode piping alloy and longer anode life. 5 Conclusions The degradation of materials in marine environments depends on the expected materials demands and service requirements for the component or systems. Galvanic corrosion is dependent on a number of factors. Understanding these factors can optimize the ability of modelling to predict galvanic interactions in various environments and under various operating conditions.
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