Required Road Salt Protection 2

[MadMango]

I deal with equipment that is used in the commercial transit field (buses and trains). Many of our components are mild steel with a powder coat finish. These usually have to pass a 100 hour or sometimes even 1000 hour salt-spray test, which isn't the problem. The problems come from the field after our equipment has been installed, and the painted finish is compromised by road debris.

Chips expose the mild steel, and road salts take their toll. We have looked into zinc plating, but these aren't durable. I was hoping that by posting in this forum, I might be lead to some other finishes that may hold up under the conditions I have detailed. Service lift of our products is on the order of 5-7 years, but may be as long as 10-15 years.

Thanks "The attempt and not the deed confounds us."

 

[CoryPad]

Modern passenger vehicles use several coating technologies for carbon steel parts exposed to salt spray conditions. One popular method for structural parts is cathodic electrocoating. Henkel and PPG are leaders in this field. Learn more at:

http://www.ppg.com/car_autocoat/ppgelectrocoat/aboutelectro.htm

 

[MadMango]

CoryPad, thanks for the tip.

I've researched a bit on electrocoat, and received some more feedback on certain parts we are having trouble with. In particular we have a hinged appearance part that is mild steel which is zinc plated. This finds it's home in the leading front-right corner of buses, and gets exposed to customers whenever the doors are opened. Due to it's position and non-protected components of the interior hinge, rust forms within a matter of weeks if the conditions are right.

Are there any protective solutions that will fight this corrosion issue and still give a high or semi-polished appreance? "The attempt and not the deed confounds us."

 

[TVP]

To further expand on what CoryPad said, the highest corrosion protection and resistance to impact is provided by the following:

1. Zinc phosphate pre-treatment
2. Thermoset polymer base coat
3. Polyethylene-based powder coat

The zinc phosphate is typically applied to a coating weight of 2.75 g/m². The thermoset polymer base coat is either a cathodic epoxy electrocoat or a zinc-rich thermosetting powder primer that is electrostatically applied. The polyethylene-based powder coat is then applied. The total thickness (pre-treatment + base coat + top coat) varies somewhere in the neighborhood of 250-500 micrometers.

The polyethylene powder coat provides excellent resistance to impact and chipping, while the pre-treatment & base coat provide the bulk of the corrosion resistance. Vehicle suspension springs use this type of coating to provide the best resistance to corrosion fatigue. PPG and Henkel are leading suppliers of zinc phosphate pre-treatments. PPG, BASF, and DuPont are leading suppliers of cathodic electrocoats. DuPont and Morton are leading suppliers of thermoplastic powder coats. You can find more information on these vendors and some good articles, etc. at Products Finishing magazine on-line:

http://www.pfonline.com

Also, MetoKote is an excellent custom coater whose capabilities include electrocoating, powder coating, and dual coating. You can access their website using the following link:

http://www.metokote.com/

 

[TVP]

Stainless steel instead of plated carbon steel would be a good solution to your corrosion problem. The cost difference may not be that significant if the corrosion problem is severe enough.

 

[CoryPad]

I didn't mention it in my first post, and TVP didn't mention it either, but Henkel has a competing technology to electrocoating called autophoretic coating. It applies a uniform polymer layer similar to epoxy electrocoat except it doesn't require an electric field. Instead, it "auto"matically deposits on steel that is immersed in the bath.

I agree that stainless steel would be a good choice here.

 

[25362]

Stainless steel with salt ? What about SCC ?

 

[TVP]

There are many stainless steel alloys that are not susceptible to SCC. The common austenitic stainless steels (FCC crystal structure) like 302 and 304 are the most susceptible. Ferritic, duplex, and superaustenitic grades are not prone to SCC.

 

[kenvlach]

Came across an article that supports the use of SS as put forth by TVP & CoryPad included salt spray, so looked it up on ASM site:

http://www.asminternational.org/Tem...ate=JournalAbstractDetail.cfm&AbstractID=5893

'Use of Stainless Steels in Bus Coach Structures'
Journal of Materials Engineering and Performance
Issue: 09, (06) Page: 669+ December 01 2000
by Antero Kyrolainen, Martti Vilpas, and Hannu Hanninen

Abstract
"This study focuses on weld integrity of stainless steels in bus coach applications. Safety aspects have been studied based on fracture mechanics and impact toughness testing. Fatigue resistance of welded rectangular hollow section (RHS) profiles was evaluated according to the Eurocode 3 (1992) fatigue standard. Corrosion resistance was studied by salt-spray chamber tests in a deicing salt atmosphere and by field testing for 3 years under an urban bus. The mechanical tests show that austenitic stainless steel EN 1.4310 (AISI 301) is a superior material, and a low-C 12% Cr alloyed stainless steel EN 1.4003 is also a competitive material in bus coach applications. According to the life cycle cost (LCC) calculations, stainless steels are competitive compared with carbon steels or aluminum."

 

[gdodd]

MM
In areas of your bus prone to chipping you can also apply a chip resistant PVC coating before painting like that applied to the rocker panel and lower door skin area of many motor vehicles.

This product is also available from Henkel