Recommended Coating for Steel Shafts in Chlorinated Water? 1

[Gungadin]

Howdy folks,

I have a project at a municipal water filtration plant (I'm one of the public works engineers) - the flocculator facility makes use of submerged steel shafts (4-7/16" dia.). The water is slightly chlorinated, and comes from a reservoir/lake. I'm quite ignorant when it comes to the myriad of coatings which would suit this application - what we are looking for is to minimize/eliminate maintenance on the shafts.

I'm wondering if one of the plasma/powder coating technologies is applicable and cost effective? Thanks for any input!

John

 

[NickE]

Unless there's a horrible wear condition on particular areas. You might be able to use a polymer of soem sort to coat the shafts. This coating would have to be perfectly continuous. Possibly an epoxy type coating? (wether two part or a coating that can be baked to crosslink)

nick

 

[TVP]

Can you provide some details about the end of the shafts? Gears, u-joints, splines, etc.? A high-quality polymer coating wold be adequate for the surface of the shaft, provided there are no wear surfaces. However, if there are areas of high stress/wear that are also exposed to the Cl-, then you should probably have them treated in a different way.

 

[YLWEA]

There are certainly high performance industrial coatings that are resistant to the fluids you describe, unless there is an excessive amount of abrasion. I would strongly consider supplementing the coating with sacrifical cathodic protection anodes, particlularly if shaft balance is not a factor (does it rotate? is it hollow with fluid on ID and OD?) You can get some good guidance on selection of specific coatings and application specs and inspection from one of your local coating reps. If you don't know local reps, the national offices are generally quite happy to either provide you with a local contact or work with you directly. try the NACE buyer's guide to find a list of coating manufacturer's by using the key word "coatings". You can even search for resources specifically in your state or city if you wish.

http://www.nace.org/buyersguide2002/BrowseCategory.asp?lo=165&hi=1278

In my experience with thermal sprayed metallic coatings in immersion service they can work if they are top-coated or impregnated with a sealer, but typically they are more expensive than a nonmetallic "paint-type" coating.

 

[moltenmetal]

Gungadin: are you talking about a shaft driving a thickener rake? Then the question will be: what are you coating the rake itself with, if it too is made out of carbon steel? If indeed this is a thickener rake drive, the rake will be subject to some abrasion. Just how much will depend on how thick you intend to run the underflow solids, their nature/source etc.

In low abrasion service, there are plenty of options, sandblasting and painting with a high solids epoxy being a cheap and easy method for a plain shaft. But attachment points (couplings, hubs etc.) and complex surfaces like rake tines etc. are a bit trickier. For most coatings, you need to radius any sharp corners to ensure that the coating doesn't thin at the corners.

 

[Gungadin]

Thanks guys,

The shafts do rotate slowly, the paddles are FRP, andthe liquid is lake water. The shaft sections are held together by compression couplings. The end of the shaft assembly passes through a wall (with a seal) where it is driven by a gear and chain off a motor.

What is the longevity of the epoxy coatings? I'm hoping to get 15 years or more before having to refurbish.

YLWEA thanks for the NACE references.

 

[YLWEA]

with slow rotation maintaining perfect balance is probably not a big deal and if you want 15 years life I would really strongly consider cathodic protection for the shaft. The link below takes you to a site where you can check out the options, but the ones we like are the "ribbon" anodes that can be purchased in a coil and cut to length. The core wire can be tack welded to the steel shaft and the ribbon shape gives very good current distribution. The technical staff at the company with the web site can probably help you pick the best size and material. I think we typically use the zinc ribbon anode, but magnesium is also available, I think.

http://www.farwestcorrosion.com/fwst/anodgalv/timminco01.htm

 

[corrosionman]

Find a local fibreglass company who can wrap your shaft with glass tape and a chemical resistant resin. Your shaft will be OK for 30 years at least. Regards D W

 

[EdStainless]

Use the system that is used for marine service. First use a zinc rich primer on the steel. This will be you anodic protection when the top coat fails.
Use an epoxy for the top coat. You need a very good one rated for constant wet service. 100% solids marine epoxies are expensive, but work very well.
Don't plan on 15 years. This needs to be inspected every few years. If coating failure in seen it needs to be re-coated asap or the damage will be severe.

Corrosion never sleeps, but it can be slowed down.