Rust on Trim Tab

[Ozdogg]

I have a question about rust on my port side trim tab. I keep my 2120 in a slip full time and recently the guy who scrubs the bottom noticed the rust on the trim tab. He said it was electrolysis. How would I go about in preventing that from continuing? Appreciate any thoughts on this. Thank you.

 

[Andy]

I have a question about rust on my port side trim tab. I keep my 2120 in a slip full time and recently the guy who scrubs the bottom noticed the rust on the trim tab. He said it was electrolysis. How would I go about in preventing that from continuing? Appreciate any thoughts on this. Thank you.

Trim tabs are mostly made of aluminum, and aluminum does not 'rust' so to speak, but can and does corrode. Electrolysis and/or galvanic corrosion is a common issue. It's why sacrificial 'zincs' (common name for a variety of sacrificial metal alloy anodes) are attached to underwater metal parts. They 'sacrifice' themselves to 'save' / preserve the important/expensive parts. Do your tabs not have 'zincs' /cymbal anodes attached/bolted on?
The zincs should be installed on the bare metal. If the tabs are painted, attach the zincs to clean/etched/sanded metal of the tabs;

 

[atcbill]

I second Andy's reply. I have zincs on both of my tabs. I didn't have them initially but when I had the boat in a marina for a bit, I noticed "rust" colored stains starting on the tabs. Cleaned them off and added the zincs and they never came back.

 

[Ozdogg]

Trim tabs are mostly made of aluminum, and aluminum does not 'rust' so to speak, but can and does corrode. Electrolysis and/or galvanic corrosion is a common issue. It's why sacrificial 'zincs' (common name for a variety of sacrificial metal alloy anodes) are attached to underwater metal parts. They 'sacrifice' themselves to 'save' / preserve the important/expensive parts. Do your tabs not have 'zincs' /cymbal anodes attached/bolted on?
The zincs should be installed on the bare metal. If the tabs are painted, attach the zincs to clean/etched/sanded metal of the tabs;
View attachment 41356
View attachment 41355

Andy, I do have zincs installed. The boat has been in the water for almost two years and this is the first time the rust was noticed.

 

[Andy]

Andy, I do have zincs installed. The boat has been in the water for almost two years and this is the first time the rust was noticed.

Is the diver changing the zincs, with the boat in the water? Just trying to help with 'brain-storming' at this point. Are they the original zincs from 2 years ago? Are you in salt water? When I kept boats, in the water, in salt water, I had to change the zincs (my last boat had 4 large zincs) at least once a year; sometimes, twice a year.... Also, I'm wondering if it could be an outside source of the 'staining', as it's interesting the diver used the term 'rust'... (usually meaning a reddish/brown staining). Aluminum corrosion is usually white-ish/gray-ish/powdery.... In our area our waters are full of tannic-acid which creates a brown-ish stain on any in-water fiberglass and metals. If not, I'm guessing the tabs are also painted with anti-fouling 'bottom paint'?..... If the zincs were installed on top of the paint and are not making contact on the bare metal, they would not be protecting the metal....

 

[Ozdogg]

Is the diver changing the zincs, with the boat in the water? Just trying to help with 'brain-storming' at this point. Are they the original zincs from 2 years ago? Are you in salt water? When I kept boats, in the water, in salt water, I had to change the zincs (my last boat had 4 large zincs) at least once a year; sometimes, twice a year.... Also, I'm wondering if it could be an outside source of the 'staining', as it's interesting the diver used the term 'rust'... (usually meaning a reddish/brown staining). Aluminum corrosion is usually white-ish/gray-ish/powdery.... In our area our waters are full of tannic-acid which creates a brown-ish stain on any in-water fiberglass and metals. If not, I'm guessing the tabs are also painted with anti-fouling 'bottom paint'?..... If the zincs were installed on top of the paint and are not making contact on the bare metal, they would not be protecting the metal....

Yes, he has changed the zincs with boat in the water and it's kept in salt water. The tabs and zincs are not painted. I'll take a pic and post it here.

 

[Ozdogg]

Here is the video taken of the tabs and zincs.

 

[alan walker]

Here is the video taken of the tabs and zincs.

Where are your trim tabs grounded? Make sure you measure the ground or the zincs do no good.

 

[Andy]

Here is the video taken of the tabs and zincs.

My guess is that the rust is from the 'hinge'/mounting bracket' and 'hinge-rod' in the hinge, because they are likely made of a mid-quality of stainless steel and not made of aluminum. Same as the rust we see on the stainless steel boarding ladder... It would not surprise me if your tabs are stainless steel too. Again, just brain-storming here, but if the tabs, hinges, and 'hinge-pin' is stainless steel, that would solve the mystery. Stain-LESS steel, means it just stains (rusts!) less than 'other' steel. ☺

 

[Ozdogg]

My guess is that the rust is from the 'hinge'/mounting bracket' and 'hinge-rod' in the hinge, because they are likely made of a mid-quality of stainless steel and not made of aluminum. Same as the rust we see on the stainless steel boarding ladder... It would not surprise me if your tabs are stainless steel too. Again, just brain-storming here, but if the tabs, hinges, and 'hinge-pin' is stainless steel, that would solve the mystery. Stain-LESS steel, means it just stains (rusts!) less than 'other' steel. ☺
View attachment 41358

Thank you, Andy. So, should I be worried and change the tabs out now or?

 

[Andy]

Thank you, Andy. So, should I be worried and change the tabs out now or?

Hi Ozdogg, firstly, the video you sent was super-helpful! And, from what I could see from the video, your diver is doing a great job taking care of your boat... I would not be concerned right now about the surface rust that the video shows on your tabs; I can assure you, there is no need to change out the tabs at this time. EDIT. As you are already doing, I would keep an eye on the rust over the next few seasons to see if it deteriorates beyond the 'surface' rust you have now...The surface rust that's seen on the tabs (and swim ladder) in the video is common on most marine stainless steel that sits in salt water. The most common types of marine s/s are 316, 316L and 304, 316 is considered the best of these, yet all of them can develop some surface rust over time, especially where welds are present and especially if the surface of the stainless steel gets scratched or abraded. S/S has a protective film/coating as such that helps to protects it. It's hard to keep this protective film on trim tabs as they are abused just because of where they are located and how they function. If I knew the brand of your tabs I'd be happy to find out which 'type' s/s yours are made of. Not that it matters right now, but it may be helpful when you research replacements in the future...... Side note; I have owned 20+ boats since the 1960s. (Actually over 30, but the smaller ones don't count! ☺) On all of these boats, except two, the stainless steel eventually got surface rust. Our 35' CT Sundeck-Trawler that we had for 28 years has ZERO rust on the stainless. (This is very unusual for a Taiwanese-made boat). I sold it 6 years ago, but keep up with it; still no rust. Why? I was religious about maintaining the metal with MDR Metal Polish/Protector. The other boat with ZERO surface rust on the s/s is our present boat; Parker 2520 XLD. Why no rust? I am anal about keeping the s/s coated with MDR Metal Polish/Protector. All this s/s is of course above the water line; not underwater like your tabs are. I don't know of any polish, coating, paint etc. that can stick well enough, long enough to underwater metal, to provide protection. The tabs on our Parker are aluminum, and were prepped, barrier-coated, and bottom painted; the paint does not even last the entire season!

 

[Ozdogg]

Hi Ozdogg, firstly, the video you sent was super-helpful! And, from what I could see from the video, your diver is doing a great job taking care of your boat... I would not be concerned right now about the surface rust that the video shows on your tabs; I can assure you, there is no need to change out the tabs at this time. EDIT. As you are already doing, I would keep an eye on the rust over the next few seasons to see if it deteriorates beyond the 'surface' rust you have now...The surface rust that's seen on the tabs (and swim ladder) in the video is common on most marine stainless steel that sits in salt water. The most common types of marine s/s are 316, 316L and 304, 316 is considered the best of these, yet all of them can develop some surface rust over time, especially where welds are present and especially if the surface of the stainless steel gets scratched or abraded. S/S has a protective film/coating as such that helps to protects it. It's hard to keep this protective film on trim tabs as they are abused just because of where they are located and how they function. If I knew the brand of your tabs I'd be happy to find out which 'type' s/s yours are made of. Not that it matters right now, but it may be helpful when you research replacements in the future...... Side note; I have owned 20+ boats since the 1960s. (Actually over 30, but the smaller ones don't count! ☺) On all of these boats, except two, the stainless steel eventually got surface rust. Our 35' CT Sundeck-Trawler that we had for 28 years has ZERO rust on the stainless. (This is very unusual for a Taiwanese-made boat). I sold it 6 years ago, but keep up with it; still no rust. Why? I was religious about maintaining the metal with MDR Metal Polish/Protector. The other boat with ZERO surface rust on the s/s is our present boat; Parker 2520 XLD. Why no rust? I am anal about keeping the s/s coated with MDR Metal Polish/Protector. All this s/s is of course above the water line; not underwater like your tabs are. I don't know of any polish, coating, paint etc. that can stick well enough, long enough to underwater metal, to provide protection. The tabs on our Parker are aluminum, and were prepped, barrier-coated, and bottom painted; the paint does not even last the entire season!

Thank you, Andy! This was super helpful. I don't know the brand of the tabs, but they came new with the boat when I purchased it in 2021.