What is happening here? (Corrosion)

Hi All,

I work on a ship and have noticed this recently. It is a steel bulwark (left side) that has had an aluminium section bolted into it (right section) after fire damage. Joined with stainless steel fasteners. It appears the steel plate is corroding significantly, is it the fasteners causing this as I imagine if there was interaction between the steel and the alu then the alu would corrode faster than the steel no?
The fasteners do have plastic washers under the caps but are in a poor condition.
Thoughts welcome!

 

That certainly looks like galvanic corrosion. The plastic washers are probably keeping the nuts from causing corrosion, but the bolts themselves may not be isolated from the surrounding material. Also, the surrounding material doesn't look like aluminum, judging by the look of that iron oxide. Perhaps it is bleeding under the paint. I would check the chemical composition of the paint to see if there was a metal in it that can oxidize.

Where is that located on board? There also needs to be an electrolytic (usually seawater on a boat) and a path to ground. Since that is a metal boat, there is certainly that.

-Will

 

Lots of the pilot houses on steel boats were aluminum and affixed like that, seemed like they all eventually had a galvanic corrosion issue eventually. Didn't matter how much flange isolation, paint, gasketing or washers added. Best one I saw was blasted over bored then painted with a good aluminum primer. Then bolted on with poly bushings, before being mated with poly washers.

In 2011 a guy next to me re did his with explosive welded data clad material.... it was some shockingly expensive stuff.

Seems like it's all a balance, on the houses built in the early 90s it was usually the sons of the guys 25 years later fixing the corrosion. How many years old is the segment in question? Sometimes the solution is replacement on decade intervels....

 

Look at a galvanic chart.

One of these 3 metals does not belong in the fabrication.

One of these is not like the others.

Aluminum is causing most all the mayhem when in the presence of saltwater electrolyte. Basically, you have created a small battery of sorts. The corrosion is nature’s way to equilibrium.

 

Welcome to the forums.
What you have here, IMHO, is two separate issues.

First, if the steel was fire damaged and then cut back to recieve the new aluminium plate, then I'm pretty sure what you have is fire damaged steel. You need to take samples as you cut back until you remove all of the heat affected zone. That zone has significantly higher electroactivity because of damage to the 'normal' steel grain structure and eutectic. This is why steel normally corrodes right alongside a weld, not in the weld material. Heat-affected zone - Wikipedia https://en.wikipedia.org/wiki/Heat-affected_zone

Second, you have an area problem. Yeah, the exposed stainless acorn nuts look cool, but installing them damaged the paint. Now you have a large cathode area (the acorn nut) and a small anode area (i.e. the paint scratches) because the rest of the steel and aluminium are painted. If it was a marine grade aluminium and you had left it unpainted, you most likely would just have light surface corrosion over the aluminium (i.e. large anode-small cathode). Galvanic anode - Wikipedia https://en.wikipedia.org/wiki/Galvanic_anode

Finally, one other thing to consider is a concentration cell in the bolt hole. In normal situations this would affect the SS bolt not the plate, but with damaged steel <shrug>. After making repairs I would paint everything...no water intrusion...no large cathode area. Crevice corrosion - Wikipedia https://en.wikipedia.org/wiki/Crevice_corrosion

 

I would first get a magnet and verify the patch is actually aluminum.