Counter-acting pitting in boilers, alt energies

[shbazjinkens]

In another forum I once met a person who had patented a method of resisting pitting on the outside of cylinders in an internal combustion engine. All there was to it was a teflon coating. After looking at a lot of pictures of steam engines with pitting problems, I wonder if this wouldn't be helpful? Surely if used in a boiler it could extend the useful life quite a bit, at least.

You can read specific specs of different grades of teflon at the DuPont site.

http://www.dupont.com/teflon/coatings/o ... rties.html

I would think that it might not help much inside of an engine because being pummelled with wet steam is going to pit about anything, but where there's only metal-to-water contact I could see it helping a great deal. It can be applied by a spray-on coating followed by baking.

Thoughts?

[gwrdriver]

There was a man on one of the live steam lists who insisted that if all steel model boilers were internally nickel plated that corrosion problems would be eliminated. I'm not metalurgist, and maybe in a perfect world that might be so, but my contention was that unless every molecule of surface area was plated, and stayed plated, which in the real world was unlikely, that rust would take hold at any defect or break, no matter how small, and the preventative barrier would be broken and therefore of essentially no use at all. The growing rust would progressively pop off the plating.

I would have the same thinking about teflon coating. One breach of the coating, especially if it were in an inaccessible place, would open the gates to progressive delimanition of the coating.

[shbazjinkens]

What you're saying is somewhat true, but if it was properly done it wouldn't be a problem. A lot of preparation is required for proper nickel coating. If you've ever been to a chrome shop you've seen it (chrome plating is actually nickel covered by a thin layer of chromium). The interior would need to be entirely free of rust so that the nickel could adequately bond through electrolysis (rust would naturally inhibit electrical bonds). It is feasible. You see it working all the time.

Teflon can be applied as a spray coating - so you could lay it on thick and get away with it a lot easier and with a lot less surface preparation. Some light sandblasting would probably do it. It's also very resiliant. The alternative being no coating at all, I'd definately try it out and see what happens.

Teflon coating would be much cheaper than Nickel coating, since it's mass produced and widely available.

[Marty_Knox]

I can see several problems with your suggestion. Basically it's much easier and cheaper to combat corrosion in a boiler by controlling your water chemistry. Another issue is that the teflon will cut down on the heat transfer. Also you need to consider that there is a serious health problem if teflon burns.

But bottom line is, I worked at a DuPont plant as a boilermaker - and DuPont doesn't coat the inside of their boilers with teflon, and they make the stuff!

[Harold_V]

What you're saying is somewhat true, but if it was properly done it wouldn't be a problem. A lot of preparation is required for proper nickel coating. If you've ever been to a chrome shop you've seen it (chrome plating is actually nickel covered by a thin layer of chromium).

That is not true of industrial (hard) chrome, which has nothing underneath. After proper cleaning, the chrome is applied directly to the base material.

The interior would need to be entirely free of rust so that the nickel could adequately bond through electrolysis (rust would naturally inhibit electrical bonds). It is feasible. You see it working all the time.

You are overlooking the fact that boilers are comprised of many pieces that are brought together and fastened/joined from the outside, where there are few problems with access. The part that needs protection, the interior, may have (you can read that will have) areas that would not conduct electricity for various reasons, so they would not get covered. Sand blasting alone would not be adequate, if for no other reason, lack of proper access to all corners. It's a fact that it's difficult for plating to cover inside corners, even when they conduct well, so that, too, would be a problem.

My experience with running a ball mill tells me that one can control rust simply by controlling pH, which has been practiced in boiler operation for years.

You want a corrosion proof boiler? Explore making one from Monel. The only significant problem with using it is the cost. [img]/ubb/images/graemlins/tongue.gif"%20alt="[/img]

Harold

[Andypullen]

Right on Harold, Harry and Marty.

Also, how are you going to get inside a model boiler to clean it and then coat it? Full sized boilers are hard enough to work on inside. And everything is made to be replaced at some point inside those....

Control your ph levels in the water and you will minimize your corrosion problem.

Andy Pullen

[UnkaJesse]

What about the reduction in heat transfer?

Unka(Well, no one else mentioned that aspect so I thought I would drag it out [img]/ubb/images/graemlins/tongue.gif"%20alt="[/img])Jesse

[Marty_Knox]

Who didn't mention it? Re-read my post, Jesse

[bcody]

Down at the Riverside Live Steamer's track they have two water systems in the steaming bays, one regular city water and the other is soft water. I also have a water softner built into my boat (RANEL II) so all the water that goes into the boiler or the storage tanks is treated. Bill

[BillJ]

Speaking as an industrial steam engineer only, and knowing that model practice is not the same thing:
1. Marty is right; industry uses mild steel because that's what works best. With ideal water chemistry the steel becomes coated with a protective layer of black oxide.

2. Water treatment consists of softening or demineralizing and blowdown, to reduce scale, deaerating to remove O2 and CO2, and pH balance of usually 10 - 11 depending on the metals in your boiler. Without getting into the fancy-schmancy that doesn't apply here. There's no reason that hobby sized boilers can't achieve reasonable water control.

Leaving the vent open on the boiler while you steam up until it's blowing good will drive off most of the dissolved air in the boiler. Heating the hotwell will evaporate some dissolved air while you're running. Good pH and hardness control will minimize the damage from what you can't remove. (Scale from hardness can trap and concentrate corrosive ions.)

Corrosion isn't just a reaction between oxygen and metal. If it was, steel would rust as fast in Arizona as in Florida. It's an electrochemical reaction and your boiler water is the electrolyte. So good water chemistry will extend the life of your boiler. It doesn't have to become a second hobby!

[shbazjinkens]

So I see the answer is a resounding "No."

Thanks.
[img]/ubb/images/graemlins/wink.gif"%20alt="[/img]

[Harold_V]

So I see the answer is a resounding "No."

Only because there are so many obstacles to overcome. The idea is sound, just not realistic. The interior would be no better than the weakest point, which would most likely be every joint in the boiler, where there would likely be no plating. Very little benefit for having jumped through the hoops.

Personally, if I had enough money to go that route, I'd definitely use Monel. I've priced it and it takes your breath away. For a boiler for a Northern, you'd likely spend in the area of $6,000 just for tubes. You'd still have the shell to deal with. Maybe when I win the lottery, or when Bill Gates sends us a sympathy check. [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

Harold