Galvanic corrosion between Inconel fasteners and Monel in seawater

Q: I am considering using Inconel 718 fasteners with a Monel K-500 flange in seawater. I’m concered as Inconel is the more anodic of the two and is also has a smaller surface area. Would this cause galvanic corrosion?

A: This is a great question. In this case if you plan on using Inconel 718 fasteners with Monel flanges you should be OK. Because the Inconel is the more noble, the galvanic effects will be pushed to the K500 Monel. This is where you want any galvanic effects to be on the larger mass part and not on the fasteners. Because the mass of the flanges is much larger than the bolts, the galvanic effects will be minimized. Furthermore the galvanic effects between Monel and Inconel as relatively minor. See our galvanic compatibility chart

Best bolts for Inconel 625 flanges

q: What bolting material should be used with Inconel 625 flanges? We used ASTM A193 Gr B7 however noticing some corrosion issues.

A: This is a very common problem but to answer it accurately, it depends on the environment that your flanges see and also to a certain extent what media is inside the flange.

If we focus on corrosion only, the reason why the media contained by the flange is of concern is there are normally small fugitive emissions. Depending on the corrosiveness of the medial it could certainly attack the B7 bolts given the fact that you are probably are trying to contain a fairly corrosive media by using Inconel 625. Additionally you will accelerate corrosion via galvanic effects if the bolting get wet.

Beyond corrosion, you need to consider the strength of the bolts as well. B7 has a min yield of 105ksi while Inconel 625 has a typical yield of 85ksi. So while Inconel 625 will certainly work as bolting from a corrosion point of view, you need to see if it has enough strength for you pressure rating. If you need higher strength you might want to consider Inconel 718 as it has both excellent strength and corrosion resistant properties.

Galvanic corrosion using Inconel bolts

Q: Can dissimilar (also called galvanic) corrosion occurs between Inconel bolts and regular carbon steel?

A: Anytime you have dissimilar materials you have the potential for galvanic corrosion. For this to happen the materials need to be wet or in some kind of solution. If they are dry it’s greatly reduced although humidity could be enough to cause the materials to galvanically couple.

Also whenever you have dissimilar materials you always want the part with less mass, in the case the bolts, to be the more noble alloy. Because the carbon steel plate has much more mass than the screw its affects will be greatly minimized. This is important as you don’t want to corrode the threads of the bolts.