With a few exceptions, most metals owe their corrosion resistance to a protective surface film. Erosive fluids can damage the protective film, and remove small pieces of material as well, leading to a significant increase in penetration rate. For instance, carbon steel pipe carrying water is usually protected by a film of rust and its corrosion rates are typically < 1 mm/y (or 40 mils/y). The removal of the film by erosive slurry gives corrosion rates of the order of 10 mm/y (400 mils/y) in addition to the any erosion of underlying metal[1] The damage to the protect film may be the results of the fluid-induced mechanical forces or flowing-enhanced dissolution[2－3].Meanwhile, the corrosion can cause degradation in surface properties and promote the mechanical erosion under action of the mechanical forces[4]. This conjoint action of erosion and corrosion is known as erosion-corrosion[5].erosion-corrosion encompasses a wide range of flow-induced corrosion[6]. It is also regarded as a subject within the broader area of tribo-corrosion which covers all aspects of tribologically (mainly mechanically) induced interactions with electrochemical processes[7].As summarized by Postlethwaite and Nesic[6], the sources of the various mechanical forces that cause erosion-corrosion include:
(1) Turbulent flow, fluctuating shear stress and pressure impacts.
(2) Impact of suspended solid particles.
(3) Impacts of suspended liquid droplets in high-speed gas flow.
(4) Impact of suspended gas bubbles in aqueous flow.
(5) The violent collapse of vapor bubbles following cavitation.
The five mechanical force sources mentioned above can be found in oil and gas production. The fluids to induce erosion-corrosion may be single phase like the portable water or multiphase flows such as various combinations of gas, oil, water and solid particles in petroleum industry[8].It is well known that the turbulent flow, fluctuating shear stress and pressure impacts are sources of flow accelerated corrosion in pipelines transporting oil and water[9] and the violent collapse of vapor bubbles in pumps and valves can result in cavitation-corrosion[10].A few typical problems of erosion-corrosion in oil and gas production are specifically mentioned as follows.
· The downhole components. Petroleum and mining drill bits are subjected to highly abrasive rock and high velocity fluid so that erosion-corrosion is among the most failure mechanisms of downhole components[11]. The entire downhole tubing string is exposed to erosion-corrosion, but points if radical flow diversion or construction such as pumps, downhole screens, chokes and subsurface safety valves are particularly at risk[12－13]. In the downholes of gas wells, the erosion-corrosion may result from the impingement of mixture of corrosive liquid droplets[14].
· The systems used to contain, transport and process erosive mineral slurries. This is particularly important for the oil sand industry of northern Alberta, Canada, where handling the processing of essentially silica-based sand (tar sand) results in server erosion-corrosion problems[7, 15].
· With the technique of CO2 injection for enhanced oil recovery and active exploitation of deep nature gas reservoirs containing CO2, server corrosion of carbon steel is experienced[16].In CO2-saturated environments, the FeCO3 scale may form and it can provide protection to some extent. The sand present in production fluids may damage and/or remove the protective scale, leading to erosion-corrosion[17].
· Petroleum refinery equipment components, typically, pump internals, thermo wells, piping elbows, nozzle, valves seats and guides, experience varying degrees of high temperature erosion and corrosion. The erosion-corrosion effects are predominant in fluidized catalytic crackers, delayed cokers, flexicokers, thermal crackers and vacuum distillation units[18].High temperature crude oil moving with high velocity across the tube wall surface may cause server localized damage. Such kind of damage may be related to the naphethenic acids that are highly aggressive in a temperature range from 220 ℃ to 400 ℃[19] and the high turbulence of fluid[20-21]. The material loss is increased significantly by the small amount of fine erodent in the crude oils that are extracted from bitumen of oil sand.
According to a recent survey, erosion-corrosion was rated in the top 5 most prevalent forms of corrosion damage in the oil and gas production[22] and cause an immense economic loss[23－24]. Many review articles on topic of erosion-corrosion investigation from different view angles can be found in open literature[3, 6－7, 23].In this paper, an attempt will be made to overview the progresses achieved in the evaluation of erosion-corrosion resistance of materials and the mitigation methods. The emphasis will be put on the synergistic effects in erosion-corrosion in flowing slurries.
The mechanisms of flow accelerated corrosion relate to the destructing and reforming of protect films. The protect films fall into two categories: (1) the relative thick porous diffusion barriers, formed on carbon steels (red rust) and copper alloys (cuprous oxide) and (2) the thin invisible passive films on stainless steels, nickel alloy and other passive metals like titanium[6].A spectrum of erosion-corrosion process in Table 1 was summarized by Poulson[3, 24]. Actually, this spectrum is more suitable to the metals with loose and less protective surface scale exposed to a single phase flow. The erosion-corrosion mechanisms of passive metals in flowing slurries are much more complicated than those shown in Table 1. For example, the mechanical erosion may contribute a major part of total material loss of stainless steels in marine pumping applications where solid erodent are present, even under the condition that the protect film is only partially removed[25]. A large amount of experimental data have indicated that, even if the corrosion component is very small, e.g. less than 5% of the pure mechanical erosion rate in absence of corrosion, the resulting erosion-corrosion rate may be much greater than that without corrosion[14, 26-30].With implantation of sand production controls, such as gravel-packing completion, the prone reservoirs produce still sand up to 5 pounds per thousand barrels and results in considerable material loss due to erosion-corrosion[12].Experimental evidence indicated that the corrosion due to wet CO2 might accelerate the erosion of C-Mn steel by a factor of 2~4[12]. Because of the damage and removal of protective scale caused by the sand impingement, the corrosion rate also increased significantly[12, 17].
As mentioned above, two different material loss mechanisms are involved in erosion-corrosion of metals, mechanical erosion and electrochemical corrosion. The mechanical erosion relates to plastic deformation and rupture in surface layer. Small pieces of metal are removed from the surface by various mechanical forces before being ionized. The electrochemical corrosion relates to the metal being dissolved into the slurry after it is ionized. Therefore, the total material loss rate ẇ is the sum of material loss rates caused by erosion ė and corrosion ċ,
To be more accurate, the corrosion rate is the more suitable term in the place of ‘erosion-corrosion rate’ in Table 1. The total material loss of material in corrosive fluids is normally larger than the sum of those caused by pure mechanical erosion and pure electrochemical corrosion. According to standard of ASTM G119, the pure mechanical erosion is defined as the erosion in an inert environment and the pure electrochemical corrosion is the corrosion under erosion-free condition. The additional wastages of erosion and corrosion components caused by the synergistic effects are regarded as the corrosion-enhanced erosion ėc and the erosion-enhanced corrosion ċe[31],
The erosion-corrosion mechanism is affected by all the factors which control corrosion and all the factors which affect erosion. In combination, the damage is synergistic and can be extremely aggressive. The synergism of erosion and corrosion, ṡ, is expressed as the sum of ėc and ċe[31]:
The synergism often contributes to such a large part of the total material loss[26－30, 32－34], that it cannot be ignored in service lifetime assessment in engineering. The corrosion is erosive liquid can be determined using the standard procedures that used in erosion-free condition, such as the one to measure the linear polarization resistance (ASTM G59)[35] and the one to generate the potentiodynamic curves (ASTM G5)[36]. The pure mechanical erosion rate in corrosive slurries should be conducted under the same hydrodynamic conditions under cathodic protection. ASTM G119 recommended polarizing the specimen to one volt cathodic with respect the open circuit potential to guarantee a fully protected condition. However, caution must be taken because hydrogen embrittlement may occur in some materials under the cathodic protection. Besides, the gas bubbles produced by the hydrogen evolution may affect the hydrodynamic conditions. A recent study indicated that the erosion rates under cathodic protection in the slurries prepared by dilute acidic solutions are much higher than those in neutral and alkaline slurries[37].In line with ASTM G119, the following dimensionless factors can be defined to describe the degree of synergism:
Although efforts have been made, it is still difficult build an integral model of erosion-corrosion[38－40].Because a large amount of factors are involved in the erosion-corrosion processes including the metallurgical features of material[41－44], the hydrodynamics of fluid[45－46] and flow field[47], the characteristics of erodent[48－51], the temperature[52－53] and corrosivity of media[37, 54].
During impingement, the sand degradation may result from the broken of sand particles and/or the bluntness of particle corner or edge, leading to a reduced erosion rate. If the effect of sand degradation is excluded, the erosion rate under a given hydrodynamic condition is independent of time[40, 55], The total material loss rate resulting from a cavitating liquid or impingement of liquid droplets is a function of time. There is an incubation time within which the rate of material loss is negligible. After the incubation, the material loss rate increases rapidly, reaches a peak value and then reduces to a steady value gradually[56－57].
When corrosion is controlled by the mass transfer of dissolved oxygen or in the boundary layer of the liquid at the electrolyte/electrode or diffusion of some other soluble species away from the surface[24], the corrosion rate is formulated as follows[58－62],
The non-dimensional parameters in Eq. (8) are Sherwood number Sh=Kd/D, Reynolds number Re=Ud/v and Schmidt number Sc=v/D; where α，β and γ are constants depending upon the flow conditions and the geometry of the test devices; K is the specific mass transfer coefficient, d is the specific size depending on the geometry of test device; D is the diffusion coefficient of the species of which diffusion in the boundary layer controls the corrosion process; U is the flow velocity; and v is the kinematic viscosity of the fluid.
Equation (8) was originally established in the rotating disk electrode (RDE) system based on kinetics of electrochemical reaction[63]. When the electrochemical reaction over the RDE surface is mass transfer control, α=0.791, β=0.7 and γ=0.356. Eq.(8) was extended to various systems. In a straight pipe, d in eq.(8) could be the pipe diameter. Corrosion rate ċ=KΔC, as the corrosion is dominated by the mass transfer process in the boundary layer at the electrode/electrolyte interface.ΔC is the concentration driving force or concentration drop of species within the boundary layer, of which the diffusion controls the corrosion process. Thus
Eq.(9) has been validated experimentally, such as the test data shown in Fig. 1. Dissolved oxygen is often believed to be the species in the flowing electrolyte controlling the corrosion process. If the corrosion reaction at the target surface is solely controlled by the diffusion of dissolved oxygen within the boundary layer, the corrosion rate is proportional to the limited current density ilim of dissolved oxygen[59－62] and the corrosion rate ċ is given by
where C0 the dissolved oxygen concentration in bulk liquid medium. Theoretically, η=1.Postlethwaite et al. pointed out only 2/3 of dissolved oxygen reaching the wall is used in oxidizing the iron into ferrous ions and that the rest is used in the oxidization of the ferrous ions to ferric ions close to the wall, so that η=2/3[11].
In Fig. 1, the exponent determined the flowing tailing water free of solid particle is around 0.75 and that in following slurries is about 0.55.This is because the linear relationship ċ=KΔC not always held. Generally, ċ∝Km[3, 24]. The deviation of m-value from 1 suggests the corrosion reaction is not fully under the mass transfer control, as depicted in Fig. 2[3, 8, 24]. In the flowing electrolyte free of sand, corrosion scale would form on the target surface (n>1: case 2 in Fig. 2). In flowing slurry, the impingement of solid particles would remove the corrosion scale and the activation of electrode may result from the dynamic plastic strain in the surface layer (n < 1:case 4 in Fig. 2). In real pipe system, the surface roughness can affect the β-value[58, 64].For a mass transfer-controlled corrosion reaction, the value for β may range from 0.5 to 1[6].
When the protective corrosion product scale exists on the surface, the apparent mass transport coefficient K is formulated as follows[66],
where KB and KF are the mass transport coefficients in the boundary layer and the corrosion product film, respectively. If the metal is under the passive condition, the mass transfer in the passive film will be much slower than that the liquid phase, KB≫KF, and K≈KF. If the fluid does not induce the breakdown of passive film, the corrosion of iron-based alloys is controlled by the diffusion of oxygen vacancy within the passive film[67] and hence the corrosion rate is controlled by the density and diffusion coefficient of oxygen vacancy density within the passive film[68]. If the fluid cannot destroy the passive film, a high flowing velocity can increase the dissolved oxygen supply at the electrode/electrolyte interface, leading to a reduced oxygen vacancy density in the passive film. As a result, the passive current density is likely to be reduced. If the fluid damages and/or destructs the passive film, the corrosion rate will increased dramatically[69].
The exact mechanism of protective film damage during erosion-corrosion in single-phased turbulent flow is still in doubt. There is uncertainty regarding the roles of mechanical forces and mass transfer in film disruption since both of them are directly related to turbulence intensity[6]. An industry standard, API RP-14E[70] recommends an empirical formula, originally developed from the experience in electric power industry with erosion-corrosion of carbon steel by steam condensate, to estimate the critical velocity Ue (ft/s) beyond which the corrosion rate will become unacceptable high due to onset of erosion-corrosion.
where ρF is the density of fluid in Ibft-3 and CAPI is a constant. A constant 450 is recommended for use in seawater injection systems constructed from corrosion-resistant alloys, 100 is for other materials and 150~200 for inhibited systems. The liquid jet impingement tests on API 5CT L80 12Cr steel indicate the erosion-corrosion resistance in absence of solid particle is considerably higher than that predicted by API RP-14E[69].The critical velocity is also a function of environment and system geometry[8].Efforts have been made to modify Eq.(8) to provide more universal CAPI factor, by taking the hardness of surface films into account[71]. Because the protective film (passive film or corrosion product scale) is very thin (~10 nm or less), both the theory and experimental techniques for evaluating the mechanical properties of the protective film are not well established[72]. The critical velocity Ue can be regarded as the critical condition leading to passive film breakdown and, therefore, it is useful tool to evaluate the erosion-corrosion susceptibility of materials under impingement liquid droplet suspending in high velocity gas flow[14].However, it does not relate to the corrosion rate after the passive film breakdown.
Based on the experimental observation of copper alloy tubes with a diameter of 25 mm[73], Efird[74] proposed the concept of ‘critical wall shear stress’ for film disruption.
where f is the Fanning friction factor[75] and its values for pipes with various surface roughness can be obtained from a Moody chart[76]. The concept of critical wall shear stress has been used to evaluate the performance of protective film of inhibitor in CO2 corrosion of carbon steel[77]. However, this idea was not tested to see if the concept of critical wall shear stress was applicable to other geometries[8].It has been pointed out that the wall shear stresses obtained are too low to remove the corrosion product scale from the pipe wall[6, 8, 24, 78]
The most severe erosion-corrosion problems occur under conditions of disturbed turbulent flow at sudden changes on the flowing system, such as bends, heat-exchanger-tube inlets, orifice plates, values, fittings and in turbo-machinery including pumps, compressors, turbines and propellers[6].The experimental evidence indicated that it is difficult to correlate the corrosion rate in the detached flow produced by the downstream of pipe expansion to the wall shear stress[24].In reality, there are fluctuating shear stress and pressure at the wall and the largest values are obtained quasi-cyclic bursting events close to the wall[6].It is worthy of studying the possibility that the corrosion product scale is physically removed by the stress resulting from the turbulent fluid[24].
In addition to corrosion process in fluid, the wall shear stress may cause an extra material loss in a corroding medium. It was found that the actual material loss in flowing electrolytes free of solid particle measured with weight loss method was higher than that calculated with the Faraday’s law based on the anodic current density determined by the electrochemical approach[79－80].The extra material loss is defined as non-Faraday’s material loss.
where ẇ is total material loss measured with the weight loss method and the Faraday’s material loss is equal to the corrosion rate
The non-Faraday material loss disappears as the corrosion is ceased by cathodic protection. However, it increases with increasing anodic current density and the wall shear stress (Fig. 3), suggesting it is a result of synergistic effect between the mechanical force and corrosion.
When the kinetic energy of solid particles in flowing slurry exceeds a threshold value, the particle impingement will remove a small piece of passive film and produce a crater. It will lead to a sharp rise of local corrosion current over the crate surface. Then the local current will decay with time because of repassivation[81].As a result, the corrosion current density over target surface that is impacted by slurry is no longer uniform and the average corrosion current density will depends on the rate of passive film removal and repassivation kinetics. In line with the kinetic analysis of slurry impingement, the average current density over the whole electrode surface can be expressed as[82]
where i is the local current density that is a function of the repassivation kinetics, A is the surface area of target and is the generation rate of the active surface area caused by slurry impingement.Cp and mp are the concentration (kg/m3) and average mass (kg) of solid particle, respectively, θ is the impingement angle, Acrater is the average surface area of crater produced by the individual particle impingement that can be measured from SEM image of surface impacted by the slurry. The kinetic mode and parameters of repassivation depend on the nature of target materials, as well as chemical characteristics and hydrodynamics of corrosion media, and can be determined directly using the single particle impingement or scratch test[52, 83].When the repassivation follows the bi-exponential law, as indicated by 304 stainless steel in the tap water[52],
where the second term in Eq.(15) (i1, τ1) relates to certain quickly decaying processes such as the formation of a passive film with monolayer thickness on a bared crater surface, and the third term (i2, τ2) relates to a slowly decaying process for growth of a passive film[84]; i1+i2=ipeak, ipeak is the peak response of local current density over the crate surface to the particle impingement; iS is the stable current density in the flowing water free of sand. In this case, the corrosion current density in flowing slurry is formulated as follows by inserting Eq.(17) into Eq.(16) and integrating[52]
The non-dimensional parameters are λ1=τ1Ȧe and λ2=τ2Ȧe that represent the combined effects of the hydrodynamic conditions and repassivation kinetics. An example in Figs. 4 and 5 indicates that Eq.(18) gives a good prediction to the corrosion current density of 304SS in the flowing slurries. iS can be regarded as the corrosion rate under the erosion-free condition, so that the corrosion augmentation defined by ASTM G119 is given by
When the repassivation follows the power law, as indicated by carbon steels in the slurries prepared with the borate buffer solution[52, 83]
The corrosion current density in flowing slurry and the corrosion augmentation will be formulated as[52]
where τ0 and m(0＜m＜1) are experimental con stants, the non-dimensional parameter λ0=τ0Ȧe.In the practical situations in engineering, λ0≪1. It has been demonstrated that Eq.(21) gives good prediction to the corrosion current densities of pipeline steels[52].
2013, Vol. 42 Issue (1): 1-10
2013, Vol. 42 Issue (1): 1-10