Sea water corrosion

[Vladimir]

Hi,

could you recommend me the good (best) material against corrosion in sea water? The parts are without abrasive wear. Thickness of coating about 3mm.
I would like to use ARC spray by TAFA 9000.
Thanks for your answer.

Vladimir

[Gordon]

Hi Vladimir

I was at first going to say a sacrificial aluminium coating, but then saw you wanted 3 mm thickness of coating.

Can you provide a bit more info, why 3 mm thickness? substrate? marine environment (tidal, submerged, deep low oxygen)? etc....

[Vladimir]

Hi Gordon,

Is concerned on the coating inside diameter on butterfly valve body (D=2800mm). The customer is wishing 3 mm of thickness - without explanation why! The main reason is the durability this valve without corrosion - 25 years. I think about Zn or Zn-Al (85/15). Can be problem with galvanic corrosion between the coating and closing disc (is made from brass) ? Exists some relation between coating thickness and corrosion decrease for see water and i.e. Zn?
Thanks for your reply.

[Gordon]

Hi Vladimir

I think 3 mm thickness of coating is well over the top and I have my doubts about the stability of a coating this thick on an internal diameter. If the brass forms electrical contact with zinc or aluminium type sacificial coatings, then yes some accelerated corrosion of the coating may occur. Zinc, zinc/aluminium, aluminium/magnesium and aluminium are all potentail candidates for long term protection because they give some cathodic protection to the substrate, slowly corroding away in preference to the substrate. Normally these coatings are rarely sprayed thicker than about 0.250 mm. I think my preference would be for aluminium and very much thinner than 3 mm (more like 0.5 mm). If 3 mm is needed for restoration of dimension then some trial work first as I think these coatings could be too highly stressed at this thickness. Under coat of NiAl or NiAl + 13Cr steel may provide a less stressful base. A coating of aluminium bronze could be considered, but the only trouble here is that you need to be 100% certain of no substrate contact with the environment. These coatings tend to act in the opposite manner to sacrificial coatings and will accelerate corrosion effects of the steel substrate should it be exposed to moisture or salt water. One other thought, may be coat the brass closing disc with zinc or aluminium as well to reduce or stop the possible bimetallic corrosion of the main coating.

[Vladimir]

Hi Gordon,

thanks a lot for your answer. I think that 3 mm is to much too! I try AlMg5. This is a good material against pitting corrosion. First I make the bond strenght test with samples with different coating thickness (min.0,3mm, max 2mm).
In The Sience And Engineering Of Thermal Spray Coatings from L.Pawlowski (Page 38) is an information about bond strenght for Zn and Al coatings by Schmidt and Matth?us. These data are in the Textbook from 9th ITSS (International Thermal Spray Symposium) that took place in 1980 in Netherlands. Have you got this textbook, please?
Corrosion resistance against see water I will test by DL-EPR to ISO 12732 - it looks to be the good test.

Vladimir

[Gordon]

Hi Vladimir

Sorry, I don't have that text. Got the following year

Aluminium magnesium is meant to be a little more "active" (sacrificial) than pure aluminium. It can be more of a pain to spray compared to aluminium though, I seem to remember problems with it spitting and banging a lot and being harsher on hardware, mind you that was in the early days of development.

[charlie castro]

Hi,

could you recommend me the best material against corrosion in sea water? That material must float just below the water level and or be flexible .

[Gordon]

Hi Charlie

to the Surface Engineering Forum.

(10-21-2009 05:23 AM)charlie castro Wrote:  Hi,

could you recommend me the best material against corrosion in sea water? That material must float just below the water level and or be flexible .

I think we will need much more information to enable us to give a sensible answer.