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Author Topic:   Corrosion Prevention on Lower Unit

BilgeRat posted 07-20-2010 10:01 PM ET (US)
I have just drilled the holes in the lower unit of my '98 Merc 90 to mount a Minn Kota engine mounted trolling motor with stainless steel bolts. The motor housing is made from some sort of non-metal material. Is there any sort of corrosion preventative or paint that I should use on the bare metal of the inside of the holes? Have I created some sort of electrolytic soup that will cause big problems? The boat will be used in salt water as well as fresh water.
jimh posted 07-21-2010 08:55 AM ET (US)
The aluminum components of the outboard motor are already protected against galvanic corrosion by the presence of sacrificial anodes, usually made of zinc or magnesium, or in some cases an alloy with some aluminum, which are less noble and will be more likely to become active in a galvanic reaction with other metals. The stainless steel components are more noble than aluminum and are protected by the sacrificial anodes and the aluminum components. The non-metal non-conductive housing of the electric trolling motor is not at risk for galvanic corrosion.
If you wish, you could cover the exposed bare aluminum metal in the holes of the anti-ventilation plate with paint. A good paint for aluminum is zinc-chromate primer, which has been used for decades to reduce the corrosion of bare aluminum. Recently, regulations regarding shipment of hazardous materials in some wacky states have prevented the economical distribution of zinc-chromate primer paint in the typical aerosol spray can, and instead zinc-phosphate primer paint is substituted.
BilgeRat posted 07-21-2010 09:58 AM ET (US)
Thanks....My first inclination was to paint the bare surfaces, but, given that many paints contain metals, I was uncertain that I may be adding fuel to the fire. It's good to hear the voice of experience.
sternorama posted 07-21-2010 04:37 PM ET (US)
You will probably be putting a stainless screw through that bare aluminum casting you drilled through, and that is a perfect place for galvanic corrosion of the dissimilar metals (between the CRES and the AL). Seal the bare aluminum with paint or clear nail polish or something. Coat the threads of the stainless fastener with gasket seal compound (sticky goey stuff that never really dries) or at least some grease. My 2$ worth (haven't seen the cents sign on a keyboard for awhile-must be inflation).-G
BilgeRat posted 07-21-2010 09:49 PM ET (US)
Local West Marine (Augusta, GA)has Zinc Chromate primer in spray cans. Cost for paint is about $2.80 for each of six holes...about $50 per square inch of bare metal. Shouldn't complain, though, I went looking, they were my fourth stop, and they had it. Put the right "spin" on it, and it was my lucky day.
dscew posted 07-21-2010 10:03 PM ET (US)
http://www.westmarine.com/1/1/27913-zinc-chromate-green-spray-primer. html
199213 posted 07-22-2010 06:41 AM ET (US)
I think you are misreading the label on that Moeller paint can from West Marine. That paint is a corrosion resistant etching primer made for below the water line as you expect. The color code is 25472 also known as "Zinc Chromate Green". There is some lead chromate in it, but no zinc chromate.
Jefecinco posted 07-22-2010 10:13 AM ET (US)
BilgeRat,
Unless you keep your boat on a mooring or in a slip you probably don't need to do anything to prevent corrosion.
I had a hydrofoil mounted to the aluminum A/V plate with stainless steel bolts for about ten years and had no corrosion damage. I did not use any paint or sealant in the bolt holes. The boat was used in salt, brackish and fresh water.
The boat was trailered.
Butch

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Author	Topic: Corrosion Prevention on Lower Unit
BilgeRat	posted 07-20-2010 10:01 PM ET (US) I have just drilled the holes in the lower unit of my '98 Merc 90 to mount a Minn Kota engine mounted trolling motor with stainless steel bolts. The motor housing is made from some sort of non-metal material. Is there any sort of corrosion preventative or paint that I should use on the bare metal of the inside of the holes? Have I created some sort of electrolytic soup that will cause big problems? The boat will be used in salt water as well as fresh water.
jimh	posted 07-21-2010 08:55 AM ET (US) The aluminum components of the outboard motor are already protected against galvanic corrosion by the presence of sacrificial anodes, usually made of zinc or magnesium, or in some cases an alloy with some aluminum, which are less noble and will be more likely to become active in a galvanic reaction with other metals. The stainless steel components are more noble than aluminum and are protected by the sacrificial anodes and the aluminum components. The non-metal non-conductive housing of the electric trolling motor is not at risk for galvanic corrosion. If you wish, you could cover the exposed bare aluminum metal in the holes of the anti-ventilation plate with paint. A good paint for aluminum is zinc-chromate primer, which has been used for decades to reduce the corrosion of bare aluminum. Recently, regulations regarding shipment of hazardous materials in some wacky states have prevented the economical distribution of zinc-chromate primer paint in the typical aerosol spray can, and instead zinc-phosphate primer paint is substituted.
BilgeRat	posted 07-21-2010 09:58 AM ET (US) Thanks....My first inclination was to paint the bare surfaces, but, given that many paints contain metals, I was uncertain that I may be adding fuel to the fire. It's good to hear the voice of experience.
sternorama	posted 07-21-2010 04:37 PM ET (US) You will probably be putting a stainless screw through that bare aluminum casting you drilled through, and that is a perfect place for galvanic corrosion of the dissimilar metals (between the CRES and the AL). Seal the bare aluminum with paint or clear nail polish or something. Coat the threads of the stainless fastener with gasket seal compound (sticky goey stuff that never really dries) or at least some grease. My 2$ worth (haven't seen the cents sign on a keyboard for awhile-must be inflation).-G
BilgeRat	posted 07-21-2010 09:49 PM ET (US) Local West Marine (Augusta, GA)has Zinc Chromate primer in spray cans. Cost for paint is about $2.80 for each of six holes...about $50 per square inch of bare metal. Shouldn't complain, though, I went looking, they were my fourth stop, and they had it. Put the right "spin" on it, and it was my lucky day.
dscew	posted 07-21-2010 10:03 PM ET (US) http://www.westmarine.com/1/1/27913-zinc-chromate-green-spray-primer. html
199213	posted 07-22-2010 06:41 AM ET (US) I think you are misreading the label on that Moeller paint can from West Marine. That paint is a corrosion resistant etching primer made for below the water line as you expect. The color code is 25472 also known as "Zinc Chromate Green". There is some lead chromate in it, but no zinc chromate.
Jefecinco	posted 07-22-2010 10:13 AM ET (US) BilgeRat, Unless you keep your boat on a mooring or in a slip you probably don't need to do anything to prevent corrosion. I had a hydrofoil mounted to the aluminum A/V plate with stainless steel bolts for about ten years and had no corrosion damage. I did not use any paint or sealant in the bolt holes. The boat was used in salt, brackish and fresh water. The boat was trailered. Butch