Proceedings of OMAE200726th International Conference on Offshore Mechanics and Arctic Engineering

June 10-15, 2007, San Diego, California USA

OMAE 2007-29560

The Effect of Magnetic Fields on Corrosion in Pipeline Steel

Joshua E. Jackson, Angelique N. Lasseigne-Jackson, David L. Olson, Brajendra Mishra, Meredith S. Heilig, and Jenny K. Collins

Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401

ABSTRACTMeasurements performed in earlier research have

indicated a strong effect of magnetization on hydrogen content (thermodynamics) as well as cracking, and pitting (kinetics) in pipeline steels as described in Sanchez (2005) and Sanchez et al. (2005). The effect of cold work, further increasing hydrogen content, cracking, and pitting, was also assessed. Theoretical descriptions of both thermodynamic and kinetic interpretations of the observed effect is described and correlated to observed results. There are two ways that electromagnetic current influences corrosion: (1) D/C currents (under applied or Remanent magnetic fields) and A/C electric currents (which create electromagnetic fields through Lenz’s Law) may influence magnetocorrosion as described in this paper, and (2) A/C currents also have the potential to strip the protective passive layer from materials and greatly accelerate corrosion. Electrochemical charging is currently being performed at varied magnetic field strengths to assess the nature of the observed influence of magnetization on both hydrogen content (thermodynamic) and cracking/pitting (kinetic), including the role of controlled-roll cold working. Naval and maritime use of A/C and D/C electric-powered systems including propulsion drives, servos, and controls, is increasing rapidly in sea-going operation and potential for stray currents is an increasing risk. Magnetic flux leakage inspection, using saturating magnetic fields, is widely used for reliable and accurate inspection of pipeline corrosion and wall thickness. Previous laboratory research shows a significant increase in both pipeline steel hydrogen content in steel and pitting and cracking after electrochemical hydrogen charging under an applied two Tesla magnetic field. Cold work was observed to further increase the observed effects. The thermodynamic auxiliary functions, using a derivation of Helmholtz free energy, are examined to assess the thermodynamic effects of magnetization on hydrogen content. The effect of magnetization on the

thermodynamics of electron spin configurations, interstitial solute-induced strain, magnetostriction

(directional strain induced in steel from an applied magnetic field) are considered. Kinetic interpretations of possible interactions with the Helmholtz Double (capacitor-like) Layer and the Gouy-Chapman (diffuse) layer that may lead to increased diffusion and thus to hydrogen supersaturation are described. Electrochemical impedance measurements are being performed to assess the influence of applied magnetic fields on the Helmholtz and Gouy-Chapman layers.

INTRODUCTION Strong magnetic fields, typically sufficient for magnetic saturation, are used to measure pipeline steel to observe areas of defects or wall thinning. Previous CSM laboratory research has indicated a significant increase in both hydrogen content and pitting/cracking in steel after electrochemical hydrogen charging with a two Tesla applied magnetic field compared to unmagnetized charging, as in Sanchez (2005) and Sanchez et al. (2005). A variety of sources for hydrogen exist in steel pipelines. Though used for corrosion protection, cathodic protection is also known to increase the rate of hydrogen production on steel in aqueous environments. Hydrogen solubility in pipeline steel is usually limited by equilibrium between environmental sources of hydrogen and solute hydrogen content in the steel. High hydrogen content can lead to serious damage and failures due to hydrogen-assisted cracking, especially in higher strength steels as in Braid et al. (1999). Mechanistic interpretation for hydrogen cracking processes was reviewed by Lynch (2002). Hydrogen content measurements described in Mazel (1993) in pipeline steel indicate a sixfold increase over hydrogen measured at installation and tenfold or greater after ten or more years of service. Numerous features of defects and failures due to hydrogen in pipelines with cathodic protection have been described by Shipilov and Le May (2006).

Alternating currents can lead to increased corrosion of steels by magnetocorrosion involving the current and a magnetic field induced by Lenz’s law and/or by stripping the passive layer from the surface. Shifts in the polarization curves (E vs. log I) from stray a/c and d/c currents in underground residential distribution power lines were measured by Compton (1982). Increased use of A/C and D/C electric-powered systems in naval and maritime applications requires intelligent mitigation and monitoring of stray currents and corrosion.

Observation of Magnetic Effect on Pipeline Steel Pipelines are commonly tested for defects, flaws, and wall thinning using magnetic flux leakage (MFL) inspection by smart pigging as detailed by Tiratsoo (2003). A saturating magnetic field is applied to the pipeline steel by powerful permanent magnets and sensors located between the north and south magnetic poles assess magnetic leakage from the pipe at defects, flaws, and wall thinning. The lower magnetic permeability of such areas forces the magnetic fields lines outside the pipe (leakage). Bray and Stanley (1997) describes magnetic fields on the order of 1.4 to 2.4 Tesla and 720-4,800 A/m (10.5 to 60 Oe) utilized in MFL to reach magnetic saturation of the pipe. Remanent magnetic fields in the range of 0.3 to 1.5 Tesla will remain in pipeline steel for an extended time. Magnetic assessment by vibrating sample magnetometer is very dependent on the subjective demagnetization factor arising from the demagnetizing fields arising from non-radial samples. An Epstein frame was utilized so that a closed magnetic loop can be formed, allowing precision measurements that are not subject to uncertainty from demagnetization. In Figure 2, the magnetic hysteresis loops of X70-grade pipeline steel are shown with decreasing maximum applied field. The measured magnetic saturation and remanence values, 1.9 and 1.3 Tesla respectively, are in the measured range of magnetic properties reported by Nestleroth and Crouch (1997) for 36 pipeline steels. The measured remanent magnetic fields from the hysteresis loops of Figure 1 are given as a function of maximum applied magnetic field strength in Figure 2, indicating the remanence remains above one tesla even with maximum applied field of 1, 078 A/m (13.55 Oe), which is in the lower range of MFL fields.

Figure 1. Magnetic hysteresis loops with varied Hmax of X70 pipeline steel specimen.

Figure 2. Magnetic Remanence in X70 pipeline steel as a function of Hmax.

Experimentally Observed Magnetization Effect on Hydrogen Content To assess the effect of an applied magnetic field on hydrogen content, electrochemical hydrogen charging of pipeline steel was performed in a 1N-acid solution with voltage held constant at 50 mV and current density of 500 A/m2 applied to unmagnetized steel specimens and specimens mounted between dual one-Tesla permanent magnet arrangements. The hydrogen content in grade X52, X70, and X80 (based on minimum yield strengths of 52, 70, and 80 ksi, respectively) pipeline steel specimens, shown in Figure 3, was measured by Sanchez et al. (2005) in a LECO-Hydrogen Determinator RH-404 after various hydrogen charging times with an without an applied magnetic field. Figure 3 indicates significant increases in hydrogen solubility with an applied magnetic field. The observed relative increase in hydrogen content decreases in magnitude with increasing steel strength. However, increasing steel strength is also linked to increased susceptibility to hydrogen cracking by Braid et al. (1999), so even the relatively smaller increase in hydrogen content for X80 pipeline steels with applied magnetic fields may be enough to cause significant damage.

Figure 3. Hydrogen content in X52, X70, and X80 pipeline steels as a function of hydrogen charging time with and without applied two Tesla magnetic field [Sanchez et al, 2005].

HYDROGEN SOLUBILITY ENHANCED BY MAGNETIZATION Possible thermodynamic explanations of the observed effect of magnetization on hydrogen content are examined. The Helmholtz Free Energy was selected as the proper description of the effect of magnetization on the interaction of a combination of internal and external work contributions to a thermodynamic system. The electrochemical work is external work performed on the system, as traditionally represented by the Gibbs Free Energy and Nernst equations. The magnetization influence on the electronic spin configurations is considered as an internal work contribution. The effect of magnetostriction, the directional strain induced in steel from an applied magnetic field, and interstitial solute-induced strain are considered as volume-change terms.

Hydrogenization Equilibrium Thermodynamics For a pure material system, assuming a reversible process, the change in the Helmholtz Free Energy (dA) is the negative of the total work on or by the system:

€

dA = −δωext − PdV −δωint (1)

where

€

δwext and

€

δwint are the external and internal work terms, respectively. PdV represents displacement work, and may be split into two terms in the presence of a magnetic field and solute addition, which are the internal displacement of the lattice by the magnetic field (

€

PdVMS

) and the internal strain due to solute additions (

€

PdVSol ). When considering solutions and including the contributions of chemical potentials for chemical species i, the change in the Helmholtz Free Energy is given by equation 2:

€

dA = −δwext −δwint − PdVMS − PdVSol − μ∑ini(2)

where the µini is additional free energy accounting for the addition of alloying elements to the solution (in this case, hydrogen). The region of hydrogen solubility where the appropriate reaction is given in equation 3 as:

)(.)( ][

saq HeH →+ −+

(3)

In this derivation,

€

δwint is related to magnetic effects on electron spin alignment and

€

δwext is related to the electrochemical hydrogen charging process. In a

reversible process, the external work term for the electrochemical behavior is given as:

where F is the Faraday constant (the charge of a mole of electrons) and is the electrochemical potential. The chemical potential of species i can be described by two terms:

where

€

μio is the standard chemical potential, R is the gas

constant, T is temperature, and ai is the chemical activity of each of i elements. Understanding electrochemical hydrogen in a magnetic field requires the application of the Helmholtz Free Energy (dA), an auxiliary function. For the magnetic case, the equilibrium of the Helmholtz Free Energy, dA = 0, must be considered to determine the hydrogen solubility of a metal under an applied magnetic field. Assuming equilibrium conditions and ∆A=0, solving Equation 5 gives:

Separating the natural log terms and considering that pH=-log[H] gives:

Divide by RT and solve for ln [H] (also noting that n=1):

Equation 9 gives the hydrogen solubility for the electrochemical reaction as a function of pH, temperature, electrochemical potential, and alloy contents:

where the hydrogen concentration with no magnetic field ([HB=0]) is:

€

wext = nFε (4)

€

μi = μ io + ΣiRT ln ai (5)

€

ΔA = 0 = −nFε −δwint − PdVMS − PdVSol

+ μ ioni +∑ RT ln [H]

[H +](6)

€

ΔA = 0 = −nFε −δwint − PdVMS − PdVSol

+ μ ioni∑ + RT ln[H] + 2.3(RT)pH (7)

€

ln[H] = FεRT

− δwint

RT− PdVMS

RT

− PdVSol

RT−

μ oini∑

RT− 2.3pH

(8)

€

[HB ] = [H]B= 0 exp ΔMBRT

⎡ ⎣ ⎢

⎤ ⎦ ⎥exp[PdVMS

RT]exp[PdVSol

RT](9)

Influence of Spin Magnetization on Hydrogen Content Equation 10 is the traditional expression for calculating the equilibrium hydrogen in a metal for a given ε and pH. Now consider the additional work of magnetization. Magnetic work changes the system by altering the d-band spin configuration (ΔMB) as well as by introducing strain (magnetostriction). In the presence of hydrogen there is also an additional elastic strain due to the lattice strain resulting from the incorporation of interstitial hydrogen atoms (solute-strain). The internal work on a ferrous alloy in a magnetic field is the spin magnetization, which describes the degree of alignment of the magnetic domains in the material. Sanchez (2005) proposed a spin magnetization model of equilibrium hydrogen concentration variance with magnetic spin alignment. The derived expression says that the hydrogen content will increase exponentially with increasing magnetic flux. However, calculations of the effect of one to two Tesla magnetic fields on thermodynamic driving forces indicate that the expected thermodynamic effect from the magnetization effect is on the scale of hundredths of a volt. The calculated ΔA is effectively the same as the non-magnetic field state. This previous calculation was based solely on ΔMB, which is

associated with the electronic spin states, and suggests that other work terms must be associated with the influence of the magnetic field.

Where ΔM is the change in magnetization (net alignment of the magnetic domains) and B is the magnetic field strength, combining to make ΔMB the contribution of the changed electron spin configuration. Thus, at equilibrium the Helmholtz Free Energy is a function of the Gibbs Free

Energy (non-magnetic terms) and the internal work and volume change terms related to magnetic effects as given in equation 12:

Inserting all of the contributing terms for the Gibbs Free Energy electrochemical reaction gives:

Setting dA = 0 and solving for the solubility of hydrogen (H) in the metal solid solution:

Solute-Strain Volume Change Previous research reported by Park and Olson (2000) on hydrogen effect on thermal expansion coefficient (CTE) of Invar is given in Figure 4 that indicates that small increases in hydrogen content show a significant effect on the CTE of Invar alloys. Invar has an extremely small CTE, which makes it a sensitive material for assessment of hydrogen solute-induced strain. Note that at a given temperature, such as 60 °C, the rise in CTE measurements describes the amount of strain that interstitial hydrogen is introducing into Invar.

Figure 4. CTE measurements of Invar specimens with varying hydrogen content.

Eshelby (1956) described, using a ball-in-a-hole model, the strain field associated with an interstitial atom in a lattice. He gave an equation for the “stress”, the work to move an interstitial into the lattice interstitial site. The strain of a single hydrogen atom is dependent on the difference between the volume of the interstitial site occupied with a hydrogen atom (ΩB) and the unoccupied interstitial site (ΩA):

where Ωs.f. is the volume size factor, relating the lattice elastic strain energy due to the presence of a solute atom of different size than the solvent. The (ΩB -ΩA/ ΩA) term is the volume strain ratio relative to the unoccupied strain, where ΩB -ΩA is the volume change due to the interstitial hydrogen atom. In the case of hydrogen as an interstitial solute in iron (BCC, d-band metal), the hydrogen atom gives its electron to the d-band and behaves as a localized proton between the iron atoms. The coulombic interaction

€

[H]B= 0 = exp 2.3pH[ ]exp[− μ i∑ o

ni

RT] (10)

€

dωint = ΔMB (11)

€

dA = dG − ΔMB − PdVMS − PdVSol(12)

€

dA = −nFε + ΔGo − RT ln[H +] + RT ln[H] − ΔMB − PdVMS − PdVSol(13)

€

[HB ] = [H]B= 0 exp ΔMBRT

⎡ ⎣ ⎢

⎤ ⎦ ⎥exp[−PdVMS

RT]exp[−PdVSol

RT](14)

2..

2 *.).()( fsA

ABstrain constconstcE Ω=

ΩΩ−Ω

= (15)

60 °C

of the proton with the positive core of the iron atom causes localized repulsion. The radius of the interstitial hole in BCC iron is 0.36 Å for tetrahedral sites and 0.19 Å for octahedral sites. Hydrogen atoms lead to an effective volume increase of 2.8 ± 0.2 Å in BCC tetrahedral sites and 2.1 ± 0.2 Å in BCC octahedral sites as detailed in Fukai (1993), that translates to an occupied radius of 0.87 Å in tetrahedral sites and 0.66 Å in octahedral sites. The elastic strain energy (negative in this case because energy is applied to the system) is given in the form derived by Eshelby (1956) for an ellipsoidal inclusion with uniform eigenstrain as:

where c is the solute composition, μ is the shear modulus, Ω is the mean volume per atom of the alloy, and f(c) is typically a linear function of composition for a dilute solution. Vegard’s Law is the empirical rule that a linear relation exists between concentration of constituent elements and the crystal lattice constant, at constant temperature. Expressions for calculating deviations from have been given by King (1965). Inserting Eshelby’s expression into equations 13 and 14, the effect of all the work terms on hydrogen solubility are described:

Equation 17 includes all of the discussed effects of magnetization on the thermodynamics of hydrogen in a ferrous material. The amplifying effects of the exponentials are such that magnetization only significantly alters the hydrogen solubility if the magnetostriction term is positive (promoting lattice tensile strain), as seen in equation 17. The square of the magnetic field strength in the exponential magnetostriction work term further amplifies the effect of Magnetostriction work on hydrogen solubility.

Magnetostriction Work Magnetostriction is the change in physical dimensions of a material due to an applied magnetic field. The interaction of magnetic fields with ferrous materials induces a magnetism-based strain that usually results in greater d-band electron orbital overlap and therefore a larger d-band with more electrons. Strain caused by magnetostriction may be an explanation for the excessive hydrogen content under 2 Tesla field in high-strength steel. Jaramillo et al. (2004) found that transformation temperatures and the resulting microstructures of steel are significantly changed by large applied magnetic fields during the phase transformations while cooling. An increase in hydrogen content in steel is known to produce changes in the electron d-band shape and concentration (Figure 5) due to hydrogen being an electronic donor that causes lattice strain. Hydrogen is described by Fukai

(1993) to act as an electron donor when the solvent is made primarily of elements to the right of manganese and as an electron acceptor to manganese’s left, as shown in Figure 6. This electron behavior effect has been measured using thermoelectric power (TEP) coefficient measurements by Lasseigne et al. (2004). Electron donor behavior allows the TEP measurement to be correlated to the diffusible soluble hydrogen content of metal. The effect of magnetostriction on lattice strain should also be measurable by TEP measurement.

Figure 5. Electron d-band before (top) and after (bottom) applied strain, indicating how strain changes the energy and size of d-band.

Figure 6. Hydrogen behavior as a function of elemental group in the periodic table.

In iron, magnetostriction usually is linear during the application of magnetic fields as the magnetic domains in the material align along the cube edge. The alignment results in a force which perturbs the cubic symmetry, resulting in linear distortion. The atomic separation in the direction of the magnetic moment can change as a result, typically an increase in steels. There is also a small inverse change in the size of the cross section of the ferrous material (decrease for steels), called transverse magnetostriction. Often the volume change is small. From one ferrous alloy to another, the effect can vary as much as an order of magnitude. With random distribution of the magnetic domains, the magnetostriction can create a volume increase of about 0.05 percent. Most ferromagnetic materials are sensitive to magnetostriction,

€

E strain (c) = − 23

μ 1Ω

(dΩdc

)2 f (c) (16)

€

[HB ] = [H]B= 0 exp ΔMBRT

⎡ ⎣ ⎢

⎤ ⎦ ⎥exp[±Ms

2μo2Η2

9λ s2YRT

]exp[

23

μ 1Ω

(dΩdc

)2 f (c)

RT](17)

and previous research by Sohmura and Fujita, (1980) has shown that the response of Invar materials was changed to a strongly ferromagnetic effect by electrolytic hydrogen charging. It is suggested that a strong applied magnetic field may induce increased hydrogen solubility. Uniaxial magnetic fields applied during magnetic flux testing at saturation values may lead to the magnetostriction effect and raise hydrogen solubility, as seen in equation 17 above.

where λs is the saturation magnetization constant for the material, σ is the applied stress, and θ is the angle between the domain magnetization and the stress. When dE/dθ = 0 the equilibrium position has been attained. Rearranging and solving for stress gives:

Assuming elastic work, where is the bulk modulus is a constant for specific units:

Inserting this term into the Helmholtz Free Energy:

Again setting the Helmholtz Free Energy equal to zero and solving for the hydrogen concentration gives:

KINETIC MODEL Hydrogen is absorbed in the atomic form in steels rather than as molecules. The process may be examined as a series of steps, any of which may be the rate-limiting factor. Hydrogen ions in aqueous solution must first diffuse to the surface, then reduce to hydrogen atoms, followed by absorption to the surface, and finally diffusion into the metal bulk. In environments with high hydrogen content, such as pipelines, the first step is not rate-limiting. The hydrogen ion reduction may or may not be rate-limiting. Bulk diffusion of hydrogen is extremely rapid, so normally the kinetics of hydrogen equilibrium are determined by penetration of the surface barriers. According to Fukai (1993), absorptionis the kinetic effect

which may most strongly influence hydrogen content according to Fukai (1993). At the interface between steel and the transported liquid two layers exist that limit the adsorption and transport of hydrogen ions. The Helmholtz Double Layer (HDL) is a capacitor-like separation of cation and anion charges. In additions to this compact layer, there exists a diffuse layer known as the Gouy-Chapman Layer (GCL) in which the concentration of ions near the charged surface is reduced to some degree, altering the availability of ions to be reduced and absorbed. These layers act to limit the diffusion of hydrogen content into steel and therefore limit the hydrogen content. In the event of breakdown or disturbance of these layers, increased diffusion occurs and hydrogen supersaturation may occur. Cathodic protection or an applied magnetic field leave these layers essentially undisturbed when applied separately. However, the application of cathodic protection currents in the presence of remanent magnetic fields leads to the creation of Larmor loops. In a metal, on the “effective” surface (“skin”) electrons contribute to the conductivity:

where Neff is the number of electrons in the skin layer, τ is the time of their interaction with the electromagnetic field, and m is the electron mass. The conductivity (σn) may be further split into contributions from the trapped electrons (σtrap), skipping electrons (σs), and Larmor electrons (σL) as described in Makarov (1995):

Electromagnetic waves are sinusoidally periodic in nature, and there exists two intervals of interest: when the electric field is dominant and when the magnetic field is dominant. During the period when the electric field is dominant, there exist trapped electrons, the group of electrons in which the electron trajectory lies at or near the surface region throughout the electrical field period. In the presence of small electrical and magnetic fields, these are the greatest contributors to current. Skipping electrons are electrons which move along the sample surface boundaries and collide repeatedly with the surface, and they contribute to conductivity in both time intervals according to Makarov (1995). Electromagnetic waves propagate approximately perpendicular to metal surfaces, independent of the incident angle, leading to higher surface fields. During the interval when the magnetic field is dominant, electrons move in “Larmor loops” due to the interaction of the electron charge and the magnetic field, creating a cross-product (VxB) force that create looping orbits in the conduction band electrons in the metal. Sample thickness, applied current, and external magnetic fields all contribute to the oscillating current and voltage conditions which

€

E = Eσ + E H = 32

λ sσ sin2 θ − ΗMsμ0 cos(θ0 −θ)(18)

€

σ =μ2Msμ0Η /(3λ s) (19)

€

−PdVMS = m4βMs2μ0

2Η2 /(9λ s2Y ) (20)

€

ΔA = −nFe − ΔMB + μ ioni +∑ RT ln [H]

[H +]± 4βMs

2μ02Η2 /(9λ s

2Y )(21)

€

[HB ] = [H]B= 0 exp ΔMBRT

⎡ ⎣ ⎢

⎤ ⎦ ⎥exp[±Ms

2μo2Η2

9λ s2YRT

]exp[

23

μ 1Ω

(dΩdc

)2 f (c)

RT](22)

€

σ eff =Neff e

2τme

(23)

€

σ n = σ trap + σ s + σ L (24)

occur at the surface as described in Makarov (1995). Industrial use of magnetic flux leakage and cathodic protection for field measurement and protection applications has expanded in number of application as well as strength of applied magnetic field. Figure 7 shows the effect of Larmor electrons in disturbing the metal-side of the HDL and induced Larmor electrons disturbing the electrolyte-side of the HDL and GCL.

Figure 7. Illustration of Helmholtz Double Layer and and Gouy-Chapman Layer, including Larmor Loop effects from combined electrical and magnetic fields.

Magnetism will cause Lorentz forces to stir the moving ions in the electrolyte passing within a pipeline. Sufficiently high magnetic fields, such as may be remanent after magnetic flux leakage, may create a sufficient Lorentz force to stir the electrolyte and increase the limiting exchange current density. Without any magnetic or electrical fields, corrosion occurs at a rate icorr, where the lines for hydrogen reduction and iron oxidation cross on the Evans diagram. Cathodic protection increases the rate of hydrogen production while decreasing the corrosion current. Disturbance of the HDL will increase the exchange current and thus shift the polarization curve to greater currents, increasing hydrogen content as well as the corrosion current, as shown in Figure 8.

Figure 8. Evans diagram indicating increased hydrogen production and corrosion current due to cathodic protection and Helmholtz-Layer-controlled effects on hydrogen content.

The suggested mechanism based on CSM laboratory data predicts that magnetization-enhanced corrosion damage to the pipeline steel will occur late in the pipeline coating service life. With loss of coating protection, increased cathodic protection return currents through the pipe will increase the Lorentz force (VxB) stirring phenomena if remanent magnetization is present.

A/C CORROSION EFFECTS A related corrosion phenomena has been reported from electromagnetic induction from high-power A/C power lines that parallel pipelines, causing magnetic field variation adjacent to the pipeline through Lenz’s Law, possibly resulting in HDL or GCL stirring. This phenomena was measured as shifts in the polarization curves in the E vs. log I plots by Compton (1982). Four mechanistic models have been identified and are being assessed to understand the effects of alternating current (A/C) on corrosion.

A/C Passive Layer Rectification Model Given the similarity in the hindrance of passage of current due to alternating potential in the two cases of arc welding and electrochemical corrosion, a model suggesting enhanced corrosion in A/C currents has been developed. Rectification (stripping) of the passive film may result from the influence of the cyclic potential between the pipe and adjacent electrode in the external environment. Due to lower chromium content, 13Cr Super Martensitic stainless steel is likely more susceptible than type 316 stainless steel to rectification damage to the passive film. Rectification models developed for A/C welding of passive alloys are being modified to consider how the oscillating potential in the wire relates to an external corrosion current through the electrolyte (salt water), similar to the analysis of Pokhodnaya et al. (1991). Figure 9 below illustrates the changes in welding parameters during a typical A/C welding cycle. The asymmetry of voltage and current due to A/C may potentially result in differences between the anodic and cathodic behavior in solution.

Figure 9. A/C arc parameter changes in (I) positive and (II) negative half-cycles (not including inductive effects in the circuit).

The A/C arc stability changes during each new half-cycle in Figure 9, where t1 is the time that passes before the power supplied to the electrode attains a level required for the recovery of the arc discharge, u1 is the voltage on the electrodes at moment of discharge recovery, and I1 is the current in the interelectrode space at the moment of discharge recovery. A/C Surface Layer Stirring Model Investigation of surface layer effects on corrosion under an applied magnetic field with an A/C current is being conducted. A/C fields may cause stirring of the Helmholtz and the Gouy-Chapman layers in the salt water adjacent to the stainless steel. This stirring, resulting from rapid changes of the self-generated magnetic field (Lenz’s Law), may increase the exchange current density within these layers and thus amplify the potential corrosion current.

A/C Self-Biasing Model Self-biasing of the metal surface resulting from anodic/cathodic oscillation, similar to radio-frequency (RF) self-bias during megahertz physical deposition (Figures 10) is being evaluated. In RF applications, self-biasing occurs at near-vacuum conditions. If the concept of self-biasing can be translated from behavior in RF sputtering to A/C corrosion, then a self-biasing model can be developed to explain the behavior of A/C corrosion. In aqueous environments, self-biasing (as shown in Figure 11) may potentially appear at much lower frequencies (e.g. corrosion at 60Hz). Chapman (1980) described the phenomena of self-biasing, which could potentially be modified to explain the A/C corrosion effect on the stainless steel surface.

Figure 10. Voltage and target current waveforms when a high frequency glow discharge circuit is square wave excited (Chapman, 1980).

Figure 11. Proposed voltage waveforms for self-biasing due to A/C corrosion on stainless steel pipe.

A/C Interfacial Tension Model

The effect of an alternating electric field on interfacial tension has been investigated at the metal-slag interface during welding. Relating this effect to a metal-seawater interface will be considered. A lower interfacial tension will reduce the stress intensity factor for stainless steel (Deev et al., 1980). This interfacial concept is consistent with the Griffith Crack Theory for brittle fracture.

SUMMARY The results of previous research indicate that corrosion is accelerated under the application of sufficiently large magnetic fields. Laboratory tests of magnetocorrosion indicate that saturating magnetic fields, as used in magnetic flux leakage testing, appear to increase hydrogen content and therefore the likelihood of hydrogen -related damage in older cathodic-protected

coated pipelines. Thermodynamic and kinetic models have been introduced and are being experimentally assessed to explain the observed magnetocorrosion effect. Electrochemical thermodynamics, using the Helmholtz free energy to incorporate all of the internal work, external work, and volume strain terms, was developed. Kinetic models were developed to assess the observed kinetic effects. A/C corrosion on structures due to the presence of high A/C current densities, such as those used in offshore and marine applications, is being studied. Four mechanistic models explaining the potential driving forces for A/C corrosion are presented.

Ongoing research at the Colorado School of Mines will assess the thermodynamic and kinetic effects associated with magnetocorrosion and A/C corrosion to allow determination of the causative factors.

ACKNOWLEDGEMENTS The U.S. Mineral Management Service, U.S. Pipeline and Hazardous Materials Safety Administration, and the National Institute of Standards and Technology are acknowledged for their support and guidance, with particular thanks to Dr. Ron Goldfarb of NIST-Boulder for his help with many magnetic measurements and ideas.

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