DaleH
FOUNDER of Classic Parker Forum
FYI, a friend had a stainless steel (SS) fitting installed on his brand new boat ... at the waterline, as a thru-hull exit for some overflow from the deck or bin. It was painted with your typical copper-based anti-fouling paint, the boat sat in saltwater … and now the thru-hull is an ugly corroded mess. This example prompted this post.
While a picture tells the story, see to follow, here are a few fun & easy to use facts for the layperson about this topic. Looking at the chart, you can see why zincs are use for anode protection of underwater gear. You can also see why marine bronze (alloy of copper & tin, sometimes with some other goodies added, proprietary or not) is the material of use for whenever a metal is needed underwater. There might be even better options, but the price tag for those expen$ive alloys or metals, e.g., titanium or gold, certaintly price them out of consideration.
Looking specifically at stainless steels, one can also quickly see why SS is great choice out of the water … but not the best choice for a mariner whenever it is fully immersed in water. And most SS hardware for the marine market is the 304 or equivalent series. (Note, SS can also be passivated, which adds protection, but I didn't want to bore you with too much details).
* Galvanic corrosion is corrosion that naturally occurs between dissimilar metals. Put a SS fastener in a tin boat and the aluminum boat surrounding the SS fastener or hardware will be eaten away.
* Such corrosion is exacerbated in the presence of an electrolyte (water) and even worse, when coupled with a chloride (salt from saltwater). Now you know ?why? saltwater can be sooooooo corrosive. Further evidence of this that you may have seen for yourself is older aluminum spinning reels where the paint was flaking off around the SS screws. The aluminum housing under the paint was corroding within itself, and where the paint flaked off, you may have seen a white powered corrosion (salt).
* Stainless steel is not truly stainless. The high chromium and/or nickel content makes it stain ‘less’, but no SS alloy is truly stainless.
* What causes SS to lose it’s protection? SS gets the layer of protection from the oxygen in the air, or it's passivated surface; eliminate it - like immersing it in water, or allowing water to sit on the material, or completely encapsulating the SS fastener in an enclosed hole – and the SS will corrode. An example of this is when you see ‘rust’ forming on the tops of the SS screws used to hold a SS cleat onto the f’glass washboards.
* ‘Working’ SS (machined, punched, drilled, or welded) can remove the protective oxide layer. The heat or mechanical action can disrupt the atoms at the surface and remove the protective oxide layer, leaving the area worked pretty close to be plain carbon steel. This is why Lenco trim tabs can rust and fall off – happening to more than one I know of in its 1st year of use (as the rolled hinge area is also welded).
* SS is also prone to crevice corrosion when under heavy tensile loads. That is why those ‘cheap’ SS hose clamps look so darn rusty! It is under a load and the band was pierced by a heavy metal punch machine. I recommend full-band-type of hose clamps, known as AWAB brand clamps, for any critical below-the-waterline internal connections. I've seen SS bolts holding deck hardware on that were reduced to 1/2 their full diameter, in the middle of the bolt, because the SS was fully enclosed inside the boat structure, which cuts off the oxide protective layer.
* If you need to put SS through aluminum – insulate them. They make special lubricants or protective ‘goops’ for this (Tef-Gel is but one brand), but I use adhesive-line heatshrink on the fastener body and thick nylon washers under the bolt head and nylok nuts.
I’ll add more if I missed anything, or if you guys have any questions.
While a picture tells the story, see to follow, here are a few fun & easy to use facts for the layperson about this topic. Looking at the chart, you can see why zincs are use for anode protection of underwater gear. You can also see why marine bronze (alloy of copper & tin, sometimes with some other goodies added, proprietary or not) is the material of use for whenever a metal is needed underwater. There might be even better options, but the price tag for those expen$ive alloys or metals, e.g., titanium or gold, certaintly price them out of consideration.
Looking specifically at stainless steels, one can also quickly see why SS is great choice out of the water … but not the best choice for a mariner whenever it is fully immersed in water. And most SS hardware for the marine market is the 304 or equivalent series. (Note, SS can also be passivated, which adds protection, but I didn't want to bore you with too much details).
* Galvanic corrosion is corrosion that naturally occurs between dissimilar metals. Put a SS fastener in a tin boat and the aluminum boat surrounding the SS fastener or hardware will be eaten away.
* Such corrosion is exacerbated in the presence of an electrolyte (water) and even worse, when coupled with a chloride (salt from saltwater). Now you know ?why? saltwater can be sooooooo corrosive. Further evidence of this that you may have seen for yourself is older aluminum spinning reels where the paint was flaking off around the SS screws. The aluminum housing under the paint was corroding within itself, and where the paint flaked off, you may have seen a white powered corrosion (salt).
* Stainless steel is not truly stainless. The high chromium and/or nickel content makes it stain ‘less’, but no SS alloy is truly stainless.
* What causes SS to lose it’s protection? SS gets the layer of protection from the oxygen in the air, or it's passivated surface; eliminate it - like immersing it in water, or allowing water to sit on the material, or completely encapsulating the SS fastener in an enclosed hole – and the SS will corrode. An example of this is when you see ‘rust’ forming on the tops of the SS screws used to hold a SS cleat onto the f’glass washboards.
* ‘Working’ SS (machined, punched, drilled, or welded) can remove the protective oxide layer. The heat or mechanical action can disrupt the atoms at the surface and remove the protective oxide layer, leaving the area worked pretty close to be plain carbon steel. This is why Lenco trim tabs can rust and fall off – happening to more than one I know of in its 1st year of use (as the rolled hinge area is also welded).
* SS is also prone to crevice corrosion when under heavy tensile loads. That is why those ‘cheap’ SS hose clamps look so darn rusty! It is under a load and the band was pierced by a heavy metal punch machine. I recommend full-band-type of hose clamps, known as AWAB brand clamps, for any critical below-the-waterline internal connections. I've seen SS bolts holding deck hardware on that were reduced to 1/2 their full diameter, in the middle of the bolt, because the SS was fully enclosed inside the boat structure, which cuts off the oxide protective layer.
* If you need to put SS through aluminum – insulate them. They make special lubricants or protective ‘goops’ for this (Tef-Gel is but one brand), but I use adhesive-line heatshrink on the fastener body and thick nylon washers under the bolt head and nylok nuts.
I’ll add more if I missed anything, or if you guys have any questions.
