External corrosion of api-x100 pipeline steels in near-neutral ph soils of variable aeration and nitrate ion content

The external corrosion and subsequent cracking of buried pipeline steels have been serious problems facing pipeline operators for many decades. The recently discovered near-neutral pH stress-corrosion cracking is often simulated in experimental tests by a soil solution containing dilute amounts of bicarbonate, chloride, and sulfate, deaerated with a gas mixture containing 5% carbon dioxide. However, noticeable differences in groundwater ion and oxygen content are observed between simulated and actual conditions, raising concerns about the real corrosion performance of buried pipeline steels. The present work aims to study the corrosion of a new pipeline steel grade (X100) in NS4 simulated soil solutions of different dissolved oxygen and nitrate concentrations. Variations in oxygen concentration are instigated through deaerating (5% carbon dioxide / 95% nitrogen), naturally aerating, or saturating (pure oxygen) the test solutions. Nitrate concentrations are varied between 0.005 and 0.015 M. Differential oxygen access and nitrate content promote corrosion, as deduced from open circuit potential, potentiodynamic polarization, and linear polarization resistance tests. Corrosion rates of the X100 steel are calculated using established electrochemical techniques, and a preliminary model is presented for predicting corrosion rates in solutions with oxygen and nitrate concentrations other than those tested.