Galvanic Reaction

[haggis]

Hi all,

We have an electrified monorail conveyor where the rail is aluminum and is suspended with "C" hooks made of steel. The suspension method is that the "C" hooks have a HRS plate which is bolted to the 3/8" thick web of the aluminum rail.

Traditionaly, our protection against galvanic reaction was to paint the the "C" hooks so that we had at least a film of paint between the steel and the aluminum. Stainless bolts were used to minimize the reaction at the holes.

The problem is, during a rework of the conveyor, the contractor installed the "C" hooks unpainted and there is no time to rectify this before plant start up.

My concern is, that in a humid environment during the summer months, how long could the situation remain before the structural integrity of the aluminum rail is compromised due to corrosion.

Thanks in advance for all opinions and advice.

 

[Metalguy]

It would appear that you have a very favorable anode/cathode ratio, so I think it could last a long time--from what little I know about your setup.

Why do you think the use of SS bolts helps? They probably are strong cathodes. I think you're lucky their total area is small.

 

[rnd2]

Metalguy is right. As long as there is a relatively vast surface area of aluminium compared to steel the effect of galvanic corrosion will be minimal.
For peace of mind, for a cheap and cherful solution you might consider placing a strip of electrical insulation paper between the steel and aluminium surfaces. They are dry and you won't have to paint them unless UV is a problem but you can do that insitu. Won't affect their insulation properties.
Nomex, Mylar, or Pyrolam are candidate materials. All have di-electric strengths of 300v/mil or better and compressive strengths sufficient to easily carry the load. You will need to check this out though. I reckon about 15 mil should do it.

 

[rnd2]

Correction to earlier post. Should read ....I reckon 15mil thick paper should do it....

 

[corrosionman]

Haggis the galvanic action of Stainless steel to aluminium is at least twice as great as that of Mild steel to Aluminium so you will probably have problems.Maybe you have a specified torque on the bolts so that the web is really supported by friction with the ms bracket. your galvanic action will soon allow the bolt to "eat" into the web thus reducing the grip and rearranging the design support philosophy. Easiest way out is to massively paint the bracket + bolt assenbly and prevent it ever getting wet or even damp. Regards DW

 

[haggis]

Thanks to all for the quick and informative responses. With your help and a couple of websites I have a little more insight into this situation.

Haggis

 

[rnd2]

My guess is you will only have a problem when there is a continuous film of water present, for example when it is raining, so another option is to strategically bolt sacrificial zinc anodes to the aluminium rail. The rail will not corrode as long as the zinc remains in electrical contact with the aluminium rail. Indirectly, we have years of experience with commercial aluminium boats up to 100M that operate in seawater and they rely on sacrificial zinc anodes for >20 year life.