The Effect of Magnetic Fields on Corrosion in Pipeline Steel

Abstract

Measurements 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.