Q: I’ve been using Monel bolts to attach a steel platform in a seawater situation, but have experienced some mild corrosion. Not sure why and any suggestions on a better material?
A: Monel is excellent in sea and salt water conditions. Two scenarios could have caused the corrosion. First, if the platform is located in stagnant water, this could be the culprit. Monel provides excellent protection to salt water, especially rapidly moving seawater, but it can show minor pitting corrosion in stagnant water. In this situation the pitting is typically slow after initial attack. The other issue that could be the cause is galvanic corrosion, which can occur as a result of electrolytic action in salt water. In this case because the Monel bolts are more noble than steel, its likely the steel is corroding and you may be seeing some of the corrosion production on the Monel.
Finally, if you are in stagnant water I would recommend trying either Duplex or Titanium. Duplex offers great resistance to pitting and crevice corrosion, even in brackish water. Titanium is renowned for its saltwater corrosion offering protection from both stagnant and high velocity seawater; unaffected by most marine environments including tide, splash, atmosphere and currents. Titanium not only has an edge over Duplex in terms of resistance, but also strength. Titanium grade 5 offers a yield strength of 138 ksi vs Duplex 2507‘s 82 ksi yield. Duplex though will be the more economic choice.