getting rust off of a SS propeller

[John Jones]

Over on FS someone was asking about cleaning rust off their SS prop.  I have a cheap prop that gets surface rust from just sitting no matter how hard I try to get all the salt off the boat after a trip.  I have worked my butt off with fine sandpaper, steel wool, wire brush on a 4" grinder, all with little success.


Ospho and muratic acid were suggested so I tried both.

NOTE:  FOLLOW THE SAFETY INSTRUCTIONS ON THE CONTAINERS!
I'm not to blame if you get some in your eyes or on your feet.
THE MURATIC ACID GIVES OFF VILE FUMES.  HEED THE WARNING LABEL!

Here is the prop.



Ospho from my local mom & pop hardware store.


I applied the Ospho with a  folded paper towel and of course, gloves unless you like the feel of fire ant stings.
After sitting for 15 minutes, little change.


After 15 seconds of scrubbing with a 3M Scotch Brite pad.



Next


a different blade



The rust came off instantly just by applying muratic acid with a paper towel.


Done.  Ospho does fine with a little scrubbing but muratic acid is faster.  Be sure and wash prop and lower unit thoroughly afterward.  
I suggest doing this with the motor all the way down or the acid runs inside the foot and if not washed thoroughly could eat up a seal or something.



It takes the black gunk from the exhaust off too.

I'll follow up later to see how fast it rusts again.

[fitz73222]

Now you are in my world! Stainless steel is stainless steel because of the amount of chrome. I`m not sure about alloys that are used to make propellers but I will tell you what I know. 300 series stainless usually contains about 12-18% chrome but is soft. 400 series stainless is much tougher but contains less chrome. There are many variant alloys that give good toughness and some corrosion resitance. Your exhaust sustem on your car or truck is 409 stainless which is a balance of cost and corrosion resistance. Part of your rust problem is the introduction of iron; wire brushing, brillo pads, etc are reintroducing iron into the microstructure of the stainless, causing rust. Your attempts to remove it are helping cause the problem. Aluminum oxide, common in most abrasives and sand papers are enhancing the galvanic activity when imersed in saltwater. the only way to remove those "free irons" in stainless is to have the propeller electropolished or passivated. It is an electro chemical process that removes the surface iron in all stainless steel and forms a protective oxide layer to prevent rust. In the Tampa area you can call Dixie plating for advice. They may be able to electropolish or passivate the propeller to stop the corrosion process. I`m not sure how the rubber hub in the propeller will affect the grounding process in the acidic bath. They will know if this is possible. Propeller rust also comes from galvanic stray current found in most saltwater marinas where battery chargers are used in moored boats and are not isolated do to the grounding systems on most boats. How are your zinc anodes looking these days? They are there mainly to protect the aluminum outboard but will also protect the prop from stray current. One other option is to run her a ground occasionally and just keep her shined up the old fashioned way!

[Aswaff400]

One other option is to run her a ground occasionally and just keep her shined up the old fashioned way!

thats the most common method my dad uses on his SR

[John Jones]

Thanks for the professional info.  Actually I used bronze wool instead of steel because I did know what steel wool would do to other metals.  I did not know about the effects of aluminum oxide.

I bought the prop used last year for $75.  It was really bad.  It was originally one of those black coated SS props and the original owner sanded the black coating off.  He probably initiated the problem.  One can tell just by looking at it when it's clean that it is cheap.  The welds on the quality SS Rapture prop I had on another boat were invisible but that prop never showed any signs of rust.  Not so with these welds.  They are actually a little porous looking as is the prop itself.  I bought it with the understanding that if the propeller shop could not balance it then no deal.  General Propeller serviced it for me and said it was fine.

My boat is trailered and I keep my zincs maintained.  I have two sets.  I swap them out once per year then in my spare time clean up the used set for next year, including my trolling motor zincs.

Being that it's a cheap, second-hand prop I'm not interested in spending any money on it.  I would rather put that towards a quality prop.  I really don't care so much about the looks.  Like you said, the sandbars around here usually keep it pretty clean.  ;)

[bumpster]

I may be missing something here. Why bother? Or is it along the lines of washing and waxing a car. BTW the black does not stay on forever especially if youre a backcountry flats fishing guy.

[gran398]

JJ, I use that Ospho all the time around the house. Advertised to turn iron oxide (rust) to iron sulfide? Think thats correct. Anyway, the rust turns black, then a few days later, it gets this white dust on the surface. Then you just wipe the dust off, and paint it. Works great.

Also use it to etch aluminum (jon boat) prior to painting.

[John Jones]

I may be missing something here. Why bother? Or is it along the lines of washing and waxing a car. BTW the black does not stay on forever especially if youre a backcountry flats fishing guy.

bumpster,
Why bother is pretty much my opinion.  I was just bored and curious and when I saw the thread on FS yesterday I found something to do for the day.  I already had the Ospho from working on my trailer and the muratic acid from cleaning tile.  Like I said the original owner sanded the black off.  I would have not have made the effort.  Being reliable means way more to me than looks.  Sand usually keeps mine pretty clean.  Our community launch ramp is only about a foot deep on a low tide spring tide.  ;)


Scott,
Ospho is good stuff if one is trying to protect metal.  Some day I'll make a post on making a galvanized trailer last for ever.

[John Jones]

Also use it to etch aluminum (jon boat) prior to painting.

Tide laundry detergent works pretty good for etching aluminum. ;)

[gran398]

Tide....that's neat!

Fitz, great post bro. While we're on the subject of stainless grades...what grade is used in swimming pool hardware.....I've noticed that it NEVER  has even the slightest bit of tarnish/browning, etc.

[fitz73222]

Hello Mr. Scott,
The answer to your question is........................................ 316L or 321; lots of chrome and a pinch of nickel, about the best corrosion resitance you can get and still be affordable.

fitz.

[GoneFission]

Ospho is really more for carbon steel - there are several phosphate products that are similar like Trustan and some others - kinda like bluing in a way...  

316 tubing is often used on marine and pool items, but is almost always electropolished to reduce corrosion.  It will tarnish or oxidize more quickly if the electropolished surface is broken through pitting or abrasion.  

Another idea for cleaning some alloys is ammonia - it works well to remove oxidation without the more caustic effect of acids.  Ammonia works well on aluminum and copper alloys (brass, bronze) as well.  Just put it on a let it soak or soak the item in an ammonia bath for a couple hours.  It has a smell to it, so use it outdoors.  

See ya on the water!

[gran398]

Ammonia as a latent scrub on aluminum makes sense. Thanks John for the suggestion.