anti corrosion coating

[seysie]

hi guys,

i'm pretty new to all of this (arc spraying & surface treatment)

so i have a big metal tank that has to be corrosion free, it's just protection against the elements (wind, rain, ...)

so what should i use for it and how hard should i grid blast it?
also do i have to use a bond or sealer?


i hope you guys can help me out here!


kind regards Jan

[Gordon]

Hi Jan

to the Surface Engineering Forum.

Initially, I would say a zinc thermal spray coating, assuming a relatively rural atmospheric environment and for the external surfaces (steel substrate). This coating should provide protection in a similar way as that of galvanising, giving a galvanic sacrificial coating that does not totally depend on being a 100% barrier.

A bond coat should not be necessary. Surface preparation is a critical stage in the process. the surface needs to be clean, free from corrosion products, dirt and grease. Grit blasting is needed to roughen, increase surface area and chemically activate the surface to provide a good bond to the coating. It is also important that the delay between grit blasting and coating should be as short as possible. Grit blasting needs to be complete for the area to be coated, you should not see any "shiny" areas. Once this is achieved there is no gain in over blasting the surface.

Zinc coatings in rural environments should give protection for many years without the need for any periodic maintenance. The life is dependent on zinc coating thickness. Sealer and paint systems designed for these coatings can also extend life a little and improve appearance. In a mild environment 0.003-0.004" of zinc may give you ~20 years life.

If your environment is marine, industrial or unusually acidic then aluminium coatings may be worth considering. If you are considering coating the tank internal surfaces, then knowing what the tank will contain will be very important in selecting the best coating.

Hope that helps

[seysie]

that is really helpfull, thank you!

for now it's only the outside of the tank that has to be done.

does the use of an alu-zinc coating make much difference in the quality of the product?

so in what conditions should i use a bond coat?

thx again!

[Gordon]

Hi Jan

Quote:does the use of an alu-zinc coating make much difference in the quality of the product?

A debatable question Coatings of zinc, aluminium and various alloys or co-deposition of the two are all capable to provide good protective systems. I suppose one of the main selection criteria would be the coating system cost relative to the expected or desired life span.

Zinc will have the more aggressive galvanic protective effect, which in cases where accessibility for spraying and grit blasting into corners, around brackets, areas out of site, etc., would be best as it would provide better protection for non or poorly coated areas. This is not an excuse for poor workmanship, just that real world objects don;t always provide ideal line of site access to all surfaces

Quote:so in what conditions should i use a bond coat?

I think in your situation probably none. Using a bond coat would make the coating system very much more expensive. I've only rarely come across the use of bond coats where grit blasting has not been possible or where very thick coatings have been required.

[seysie]

(11-21-2008 04:38 PM)Gordon Wrote:  Hi Jan

Quote:does the use of an alu-zinc coating make much difference in the quality of the product?

A debatable question Coatings of zinc, aluminium and various alloys or co-deposition of the two are all capable to provide good protective systems. I suppose one of the main selection criteria would be the coating system cost relative to the expected or desired life span.

Zinc will have the more aggressive galvanic protective effect, which in cases where accessibility for spraying and grit blasting into corners, around brackets, areas out of site, etc., would be best as it would provide better protection for non or poorly coated areas. This is not an excuse for poor workmanship, just that real world objects don;t always provide ideal line of site access to all surfaces

Quote:so in what conditions should i use a bond coat?

I think in your situation probably none. Using a bond coat would make the coating system very much more expensive. I've only rarely come across the use of bond coats where grit blasting has not been possible or where very thick coatings have been required.

allright this is perfect! thx for the information, it's really helpfull!


goodnight

[Stuart Milton]

Agreed with all of Gordons comments.

Blasting is absolutely key to obtaining a good quality coating. Depth of profile is not always as important as the quality of profile. You should be aim for a rough, angular profile. Typical blasting media would be chilled iron grit, grade G17 or G24 or often a mixture of the 2 are used. You can also use Garnet, Copper Slag or Aluminium Oxide, depending on your specific application.

With Zinc spraying, use of a sealer coat will extend the life of the coating. So you may for example, put on a sealed coating 100 microns thick, sealed with an epoxy sealer and get circa 10-15 years life to first maintenance. An unsealed coating would need to be around 125-150 microns as the corrosion rate of unsealed zinc is a bit higher, depending on the environment that it is in. You can seek independent guidance on coating life in BS EN ISO 14713.

Bond coats are not normally used in anti-corrosion coatings as both zinc and aluminium normaly adhere well enough for most applications.

If you need any further assistance or equipment or materials, please contact us as below.

Good luck.

[tonysemple]

Try http://www.bromoco.co.uk

[sunnyshop]

I got alot of information from there... i am new and i sure i will increase my knowledge alot