Evaluation of High Phosphorus Electroless Nickel Platings to Protect Carbon Steel Heat Exchangers in Seawater Service

The purpose of this project was to determine the suitability of using a high phosphorus electroless nickel (HPEN) coating to prevent corrosion in carbon steel heat exchangers in seawater service. 1 in. x 4 in. samples were cut from ¼ in. SA-516-70 pressure vessel plate, pickled to remove mill scale, dressed with 300 grit sand paper and sent out for plating with 2 mils of HPEN. Due to a clerical error during plating, samples were plated to a significantly lesser extent than requested. Samples were plated with approximately 0.6 mils of HPEN versus the 2 mils requested, however this was not expected to appreciably affect the results of testing. Samples were damaged with a 60° single point cutting tool to a depth of 0.005 in. and exposed to 98.6°F aerated seawater in accordance with ASTM G31-72 for 24 hours, 168 hours (1 week), 672 hours (1 month) and 1000 hrs. Samples were removed from the corrodent, vacuum infiltrated with epoxy and prepared for metallographic analysis. Both depth of attack and material loss over time were measured, and it was found that the galvanic couple established through the damaged plating greatly accelerated attack. Attack was accelerated to such a degree that after 1000 hours, corrosion had proceeded 0.029 in. past the original path of the tool tip in the damaged samples versus only 0.0018 in. for the uncoated sample. Graphing material loss versus time reveals linear attack with a coefficient of determination of .9971, indicating that the attack does not slow over time. The linear nature of the material loss over time suggests that damage this extensive does not tend to plug with corrosion products slowing attack. Additionally, due to the strong metallic bond between the steel base material and the HPEN plating, it was seen that damage under the plating was difficult to observe through external visual examination. These two factors lead HPEN plating to be seen as an unsuitable means of protecting carbon steel heat exchangers in seawater service.