Powder Coating Props on BIII

[Dave S]

DaveS, Thanks for taking the time and giving your experience and observations. one quick question! Are those Anode Pucks on your Transom Bonded to the Engine or Battery Circuit of your boat. If not they are useless at best.
It's obvious that your Lake is a Harsh environment, If it is a "Common" Occurrence for BIII's to show Galvanic Corrosion of the Aluminum there will be a "Common" Cure. On my boat, 1997 Dancer,7.4/300HP with a BIII I have installed the recommended "Second" Cathod Amplifier or as Mercury refers to it a "Mercathode". Your boat is delivered with one in place, it is either blue or black. Dave, you and everybody on your Lake need to double up the Mercathode now! No need to add the Anode portion just the Blue or Black Four Wire Unit.
Also, all those little Stainless Steel Ground Bond wires on the Drive must be in Tip Top condition.
Getting back to our boat. I also painted the Gimbal and Drive. I used Dupont Imron, which is a Two Part, or Catalyzed coating system widely used in Aviation, Marine and Fleet Applications. Why? Because it is my belief that it does a better job of "Insulating" the Drive than the Mercury recommended Tin based coating. And, for guys like DaveS and I where appearance is important you can TopCoat with Clear for that Deep Black Shine! (I like it!).
Also, an area at least 1" from the Gimbal Housing on your Hull must be free of Bottom Paint. Al W.
Here's more: http://www.sterndrives.com/bravo_3_corrosion.html

Mercury also has a heavy duty Red Controller with pucks that are unique to that controller. In fact you cannot buy the pucks separately because I checked on that. That's what the dealer is installing now at the recommendation of Mercury who has supplied the components. And my entire bonding system has been thoroughly checked out and there was one wire inside the boat they repaired as a result of checking this out.

As far as paint goes, I used several coats of Zinc Chromate primer and then Mercury Phantom Black. I have to do all my work in the dealers yard and I don't have a spray gun so the Dupont Imron is probably not an option for me unless it comes in spray cans.

Your comment that every one should have the second controller is indeed right on. Those pucks do protect the drive somewhat in our lake, just not thoroughly enough. Nothing is corroded on my upper drive whereas other ones I have looked at were corroded all around the stainless nuts and bolts and elsewhere on the upper unit. I am hoping this "heavy duty" controller will work better but I am not holding my breath. If they don't work, I will continue going back to Mercury and looking for a solution. Maybe I will ask them to coat my props with whatever they use on the Sea Core drives.:thumbsup:

 

[Stray Cat]

I know of a '95 330DA owner that uses a self etching aerosol primer No. 39673 made by SEM Products on his B3's. He's used this for several years with excellent results in fresh water using magnesium anodes.

Here's a link: http://www.sem.ws/product.php?product_id=139

I found a can at a local Carquest auto parts store. I thought I would give it a try thinking after 2 season I would have some touching up to do...nope both drives show no signs of corrosion. I would estimate the annodes are half used up.

 

[AndersonAcres]

How can electrolysis and galvanic corrosion be thought of as two distinct problems? Doesn't one cause the other?

Here's the way I found it explained on one site this morning: "Electrolysis is the chemical reaction that takes place in an electrolyte between two dissimilar metals. Galvanic corrosion to the less noble metal is the result."

 

[Henry Boyd]

Ok, Let me weigh in here with the 'corrosion for dummies' diseration.

One - all corrosion, and chemistry for that matter, is about electricity. Dissimilar metals in a moist environement create an electrical circuit. Electrons jump from one molecule to another and voila iron turns to iron oxide (rust + oxygin) because the bond between the hydrogen and oxygin in the water (the H2O) is weaker than the attraction between the oxegin and the electron charged steel.

This process can be prevented in a number of ways. One way that we have discussed is to use a 'less noble' sacrifical anode, a.k.a zinc. The electric circuit is in effect re-wired from between the SS and ALuminum to between the SS and the Zincs. In doing so the zincs turn to zinc oxide. The Mercathode system is also an attempt at re-wiring the corrosion cell by introducing a actual electrical circuit.

The other manner in which corrosion can be prevented is via a coating (e.g. paint). The coating acts as a barrier, or in effect an insulator. Coatings are not always enough because at the microscopic level there is a transfer through the coating by osmosis.

In heavy duty marine coatings (e.g. ship hull paints) the sacrifical anode is ground up and disbursed in the primer. Here the purpose is a little different. The primer with zinc will prevent undercutting of the top coat when the paint is abraded by sacrificing itself over the steel in the hull. This improves the performance of the barrier coating. Galvanizing is a combination of using the zinc as a sacrificial and as a barrier.

Note that there is a third process using corrosion resistant steels, e.g. USS Corten, SSAB Domex/Dolcol W etc. These function by adding additional atoms of stuff to the steel in effect blocking the number of places the free electrons can latch on to.

Would coating the SS prop work? Yes in theory, because it insulates the SS from the environment. IN effect just like the plastic coating around a wire. Ditto for coating any and all exposed ALU on the drive housing. And by coating this could be anything from powder coating to a thick coat of grease. But we have to consider the mechanical requirements and need for longevity, so a powder coating would probably work better than paint, or grease.

Why did Gary's powder coated aluminum frames fail? Because the powder coating is still porous on the microscopic level and will allow oxegin and water through. By the way this will also happen with the SS props only the results will not be obvious because the props do not degrade. Powder coating will not be a perfect insulator, but may be better than nothing. Will this be a solution? We don't know because at this stage we do not know the relative impact on the galvanic circuit.

Hope this helps

Henry

ps how about a spell/tping checker as an add on to the message board??????

 

[Dave S]

Great explanation Henry! :thumbsup:

Here's another question for anyone who can answer it. I thoroughly cleaned my out drive today. Simple Green, brushes, scrubbing pads and lots of elbow grease got the drive to look like this.

My next step is clean all the corrosion, acid etch the metal, prime and paint. Now for the questions. Can anyone tell me why the lower unit is dull? By way of an explanation, this is the part that I painted the last time. I used Zinc Chromate Primer and Mercury Phantom Black. When I finished the paint job the last time, this part of the drive was nice and shiny just like the rest of the drive (which still has the factory paint on it). I just don't know why this part is now dull. Anyone have any ideas? I wonder if the Zinc Chromate Primer has anything to do with it since it is a self etching primer?

 

[wickeral]

DaveS, I too would like to Thank Henry for his obviously knowledgeable in-site! Thanks Henry! Maybe Henry will tone in on your question as to why you lost the Gloss on the Lower Portion of your Drive. I think you answered it already with the application of Zinc Oxide Primer. As Henry said, there is always some intrusion threw a given surface. As I previously reported I do not like Mercury Marines suggestion of using bottom paint containing "Tin". I do use a Highly Ablative Bottom Coat but keep it 1" away from the Gumbel Housing. This Thread has been a good learning curve for all of us. Al W.

 

[wickeral]

Great explanation Henry! :thumbsup:

Here's another question for anyone who can answer it. I thoroughly cleaned my out drive today. Simple Green, brushes, scrubbing pads and lots of elbow grease got the drive to look like this.

My next step is clean all the corrosion, acid etch the metal, prime and paint. Now for the questions. Can anyone tell me why the lower unit is dull? By way of an explanation, this is the part that I painted the last time. I used Zinc Chromate Primer and Mercury Phantom Black. When I finished the paint job the last time, this part of the drive was nice and shiny just like the rest of the drive (which still has the factory paint on it). I just don't know why this part is now dull. Anyone have any ideas? I wonder if the Zinc Chromate Primer has anything to do with it since it is a self etching primer?

DaveS, I just noticed that on your photos there is no Hockey Puck showing on the Trim Tabs. Just for info. purpose only, I run small Pucks on my Trim Tabs. My thoughts are that the Tabs are Bonded to the boat grounds, therefore Zinc or other Anode is needed. And, Yes, they do corrode but a noticeably slower rate. Al W.

 

[wickeral]

How can electrolysis and galvanic corrosion be thought of as two distinct problems? Doesn't one cause the other?

Here's the way I found it explained on one site this morning: "Electrolysis is the chemical reaction that takes place in an electrolyte between two dissimilar metals. Galvanic corrosion to the less noble metal is the result."

Hello Eric, Your question pertains to the process used in metal plating. I suggest you Google "Chrome Plating" for an answer that will be easier to comprehend than I could write. Al W.

 

[LtCdr. Augs]

Perhaps because the lower unit, being subject to the stream of current below the hull, effects the coating negatively as a result of that location? Compared to the upper half, which is shadowed by the stern gun'le?

I notice your lowermost skeg is abraided somewhat...

SandBlasterAugs

 

[Presentation]

Dave,

Because Mercury bakes the coating on the drive in a high temperature process.

 

[Dave S]

I notice your lowermost skeg is abraided somewhat...

SandBlasterAugs

That's due to months of low lake water and creeping out of the Marina with the drived trimmed up to avoid the shallow areas. No matter how carefull I was, the outdrive skeg dragged thru the mud a bit occasionaly.

 

[Dave S]

DaveS, I just noticed that on your photos there is no Hockey Puck showing on the Trim Tabs. Just for info. purpose only, I run small Pucks on my Trim Tabs. My thoughts are that the Tabs are Bonded to the boat grounds, therefore Zinc or other Anode is needed. And, Yes, they do corrode but a noticeably slower rate. Al W.

They are there behind that brace you see on the trim tab and therefore they don't show in the picture.

 

[Presentation]

Is the aluminum drive housing behind the prop going to corrode any less with an aluminum prop than a stainless one all other factors being the same?

Yes


Here is the problem. No such thing as an aluminum propeller for a Bravo III drive.

Combine this with the fact that you have not one, but two stainless steel propellers and bingo, your aluminum drive just became an anode.

OK, here is the “Mercury Marine” comeback. “But…since the aluminum drive is connected to zinc anodes the process will transfer to the least noble item”

Here is where this beaks down. The electrolyte, in this case “Fresh water” is less conductive then salt water. Fresh water is not a little bit less conductive then salt water, it’s a lot less conductive then fresh water.

OK, so Merc counters – “So let’s use an anode that is even less noble. Let’s use a magnesium anode.”

The issue is fresh water is such a poor conductor of electricity that the distance between the location where the stainless steel propellers meet the closest magnesium anode is too great.


**********************************
************Dave ******************
*** Dave – you say the anode hanging off the propellers is magnesium. Please check this again.

Looking at the anode at the propellers it just does not look like a corroded magnesium anode. It looks like a corroded aluminum alloy anode. Wish I could feel it in my hand to be sure.

Given your corrosion location this is your most important anode.

Or better yet, go back to your receipt and please post the part number of the anode connected to the propellers. It doesn’t look right. It looks too big to be magnesium.

Also, if you were to change out one anode more often then the rest, this would be the one to pick.

Fresh water dramatically reduces the effectiveness of the merc cathode system. You need a sealed outdrive. At the start of the season it looks like you had this. Unfortunately you need to do the process again. You also need a full set of fresh magnesium anodes every 6-8 months.

 

[Searay Jim]

Dave,

For what it's worth, my lower still shines like you describe, and all I did was spray over the old paint (and chips) with the Merc spray can. This year the qtr. sized chips on either side didn't even come back. Now it doesn't look great when you look close, because I've sprayed over the paint several times cleaning it up. If I would have sanded it, it may look much better.

I've never seen that on a lower that's been painted. I can't imagine how the whole thing ended up dull like that.

 

[Henry Boyd]

I think the dulling of the lower part of the drive is a result of plain old friction and 'wet sanding' by particles of crud in the water. The area that is dull is the area that is in the high speed stream when the boat is moving.

I noticed a link to chrome plating. That as well as anodizing is the same as corrosion, the difference being the chemistry involved.

Henry

 

[Dave S]

I refinished the outdrive today and installed new Magnesium Anodes. I will let you know what she looks like 6 months from now if we ever get enough rain to bring our lake levels back up and I can put the boat back in the water. :smt089

The refinished drive.

 

[wickeral]

Looks Beautiful! How about a shot of the whole boat? Al W.

 

[Dave S]

Looks Beautiful! How about a shot of the whole boat? Al W.

http://clubsearay.com/photoplog/index.php?u=402

 

[Searay Jim]

I think the dulling of the lower part of the drive is a result of plain old friction and 'wet sanding' by particles of crud in the water. The area that is dull is the area that is in the high speed stream when the boat is moving.

I noticed a link to chrome plating. That as well as anodizing is the same as corrosion, the difference being the chemistry involved.

Henry

If that was the case, my "just spray it on" method of painting my lower would show tremendous "sand blasting". I cruise at anywhere from 30 - 50+ and run short term 60+ and have zero dulling at all.

It looks like it's the entire surface of the lower only which is odd. At speed, the whole lower isn't submerged completely. My guess is something to do with the way it was painted or prepped.

 

[skibum]

Did I just read that a 240 can do 60MPH?

I see that you've got an "unusual" powerplant, so no doubt the HP is there. I just had no idea the hull would hande that kind of speed.

Michael