Abstract
The present work discusses the effect of hypo-eutectic, near-eutectic and hyper-eutectic electroless NiP coatings, laid over an API (American Petroleum Institute) X70 grade steel, on their resistance to H-permeation as well as corrosion against chloride attack. The H-permeation was investigated using a Devanathan-Stachursky electrochemical permeation cell. All the three NiP coatings displayed excellent H-barrier property along with good corrosion resistance in comparison to the base metal and even electrodeposited Ni coating. Detailed microstructural investigation using Scanning Electron Microscopy, Transmission Electron Microscopy, X-Ray Diffraction, X-ray Photoelectron Spectroscopy and Differential Scanning Calorimetry revealed the coatings to be primarily amorphous with the presence of a few nano-crystalline areas. The corrosion resistance of the coated steel was also investigated using both dynamic polarization test in freely aerated 3.5 wt% NaCl solution and salt fog exposure test in 5 wt% NaCl solution. Further, the corrosion products formed on these coated surfaces after prolonged salt-fog exposure were characterized with the help of Raman Spectroscopy and subsequently, correlated with their corrosion behavior. The near-eutectic NiP coating displayed higher resistance to corrosion as compared to the hypo- and hyper-eutectic NiP coatings due to the formation of amorphous Fe(OH)3 and higher α/γ FeOOH ratio in the rust.
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