Jump to content

Vactan - has it worked?!


YSA

Featured Posts

Hi all!

 

I'm in the process of getting my stern gland bilge in good shape and after getting rid of loose rust and degreasing, I applied a coat of vactan. 

Picture of what the result is attached.

 

I can't tell whether it's still rusty or if I'm good to go ahead with a top coat! 

IMG_20230525_132453.jpg

Link to comment
Share on other sites

There's no flaking - I've gone over with a scraper and hit with a hammer and nothings coming off. There are still some rust spots though, now that I've looked at it again, so will go over those bits one more time. 

 

And will make sure to use a primer, thank you! 

Link to comment
Share on other sites

duuno? is there much to be gained from a zinc primer when Vactan has dried to produce a barrier over the steel???

Give it a couple of coats of Danboline, but note that rust converters are only a third best solution so don't expect it to last for too long.

Link to comment
Share on other sites

7 hours ago, dmr said:

duuno? is there much to be gained from a zinc primer when Vactan has dried to produce a barrier over the steel???

Give it a couple of coats of Danboline, but note that rust converters are only a third best solution so don't expect it to last for too long.

 

'Zinc rich' has two meanings. One is high loadings of zinc metal powder, in a paint such as Galvafroid or Zinga which provide galvanic protection. For this to work you need direct contact between the steel and the paint, so it won't work over rust or a layer of Vactan. The other meaning is high loadings of zinc phosphate pigment which is a very good barrier against oxygen and moisture. This is the type you would want to apply over Vactan.

 

Alec

Link to comment
Share on other sites

13 minutes ago, agg221 said:

 

'Zinc rich' has two meanings. One is high loadings of zinc metal powder, in a paint such as Galvafroid or Zinga which provide galvanic protection. For this to work you need direct contact between the steel and the paint, so it won't work over rust or a layer of Vactan. The other meaning is high loadings of zinc phosphate pigment which is a very good barrier against oxygen and moisture. This is the type you would want to apply over Vactan.

 

Alec

 

This does not really add up. Almost all metal primers are rich in Zinc Phosphate and we are constantly told here that these are not moisture resistant and must be over-coated asap (except BondaPrimer). Sounds like you might know about this so please explain?

Link to comment
Share on other sites

8 hours ago, dmr said:

 but note that rust converters are only a third best solution so don't expect it to last for too long.

 

∆∆∆

The only reason so many people use Vactan is because they don't know how to prep the steel properly. Getting rid of all the rust mechanically including the rust in the bottom of pits and then priming or epoxying it is the proper way to do it and much better than converting the rust under a layer of water soluble latex.

Link to comment
Share on other sites

I've used Vactan on the welds to my roof handrail  Didn't overcoat for two years and it didn't rust anymore. I also coated the galvanised chains holding the button and rear fender. These had started to go rusty. Again, no further rust.

 

Funny how some of these materials work for some and not others!

  • Greenie 1
Link to comment
Share on other sites

On 25/05/2023 at 22:00, dmr said:

 

This does not really add up. Almost all metal primers are rich in Zinc Phosphate and we are constantly told here that these are not moisture resistant and must be over-coated asap (except BondaPrimer). Sounds like you might know about this so please explain?

Apologies for the delayed reply. The BCN Challenge lies between your post and this answer.

 

There is no unequivocal answer to this but the options as to why zinc phosphate works are very nicely set out in the introduction to this paper:

 

Yawei Shao, Cao Jia, Guozhe Meng, Tao Zhang, Fuhui Wang,
The role of a zinc phosphate pigment in the corrosion of scratched epoxy-coated steel,
Corrosion Science,
Volume 51, Issue 2,
2009,
Pages 371-379,
ISSN 0010-938X,
https://doi.org/10.1016/j.corsci.2008.11.015.
(https://www.sciencedirect.com/science/article/pii/S0010938X08004964)

 

'Generally, inhibitive pigments are introduced into organic coatings as the main substance for corrosion inhibition. For this purpose, zinc phosphate is commonly used because it is known as a green pigment in contrary to toxic pigments like chromate pigments [1], [2], [3], [4], [5], [6], [7], [8]. But there are many arguments on the roles of zinc phosphate pigment in the corrosion process of coatings in the literatures. For the first, some authors [6], [7], [8], [9], [10] revealed that the action mechanism of zinc phosphate was the phosphatisation of the iron surface and the formation of compounds with carboxyl and hydroxyl groups of the binder agent. For the second, del Amo [11] and Beiro [12] et al. believed that the addition of zinc phosphate to the paints leads to a clear improvement of their barrier function. Furthermore, Rossenbeck [13], [14] revealed that zinc phosphates improve the barrier properties and Zn-hydroxides, phosphates absorbed on the iron surface can inhibit the cathodic de-adhesion process at the interface. However, Zubielewicz [5] and Bastos [15] considered that zinc phosphate neither acted in compliance with electrochemical mechanism nor improved the barrier function of the binder, and they further believed that the addition of zinc phosphate did not exhibit any positive effect on the corrosion performance of the coatings.'

 

Personally, I think the protective role of zinc plating is a reasonably good analogy. Zinc plating acts as a barrier but if it is breached it provides protection to the steel as a sacrificial anode. However, the zinc plating also naturally oxidises so it has a finite life which is reduced if the environment is more aggressive (salt corrosion for example) or extended if a barrier is applied over the top, such as chromating (which you should not do any more) or paint (disregarding the poor adhesion without preparation).

 

The equivalent effect with zinc phosphate appears to be a combination of reaction with exposed iron, forming a barrier at the interface with the steel and an element of self-healing where because the zinc phosphate does not 'go off' it remains available to form a reactive zinc/iron phosphate barrier layer when the iron is exposed. However, if instead it reacts with the atmosphere it is 'consumed', so its barrier properties are used up by chemical reaction with the atmosphere and it no longer provides protection. That would be the case whether the zinc phosphate primer was applied directly to bare steel, immediately forming the reaction product, or applied over a pre-treated surface to remain available to react with any iron which was subsequently exposed either through mechanical damage or chemical degradation. Either way, if moisture permeates through the primer then the positive effect of the zinc phosphate is quickly 'used up' whereas if it is topcoated, it provides long term protection against the much slower ingress of water through the topcoat, either through permeation or through defects.

 

Hope that makes sense.

 

Alec

Link to comment
Share on other sites

Boating is much more important than posting on the forum, and this is especially true of the BCN challenge. We have crewed for Goliath a couple of times and so have so very good memories.

 

The suggested functioning of zinc phosphate primers is very interesting.

This leads to two questions...

Why are most primers not waterproof (assuming this true) whilst BondaPrimer is waterproof and appears to work as well, if not better, than other primers?

Why is a bit of Zinc Phosphate not added to all paints, especially undercoats (at least where this does not compromise the colour)?

Link to comment
Share on other sites

I have heard it said that no paint, be it primer, undercoat or top coat, is actually waterproof, but what companies do is to introduce loads of small particles into primers so as to massively increase the distance water has to travel to finally get under that product. The analogy was to dry stone walling.

Link to comment
Share on other sites

17 minutes ago, Col_T said:

I have heard it said that no paint, be it primer, undercoat or top coat, is actually waterproof, but what companies do is to introduce loads of small particles into primers so as to massively increase the distance water has to travel to finally get under that product. The analogy was to dry stone walling.

and I think that is truer today than 25 years ago

Link to comment
Share on other sites

On 26/05/2023 at 12:59, Old Son said:

I've used Vactan on the welds to my roof handrail  Didn't overcoat for two years and it didn't rust anymore. I also coated the galvanised chains holding the button and rear fender. These had started to go rusty. Again, no further rust.

 

Funny how some of these materials work for some and not others!

 

So it's worked for 2 years? That doesn't sound very long to me.

Link to comment
Share on other sites

21 hours ago, dmr said:

Boating is much more important than posting on the forum, and this is especially true of the BCN challenge. We have crewed for Goliath a couple of times and so have so very good memories.

 

The suggested functioning of zinc phosphate primers is very interesting.

This leads to two questions...

Why are most primers not waterproof (assuming this true) whilst BondaPrimer is waterproof and appears to work as well, if not better, than other primers?

Why is a bit of Zinc Phosphate not added to all paints, especially undercoats (at least where this does not compromise the colour)?

It was a good BCN Challenge with a surprisingly good result for us, given that we are neither a boat nor a crew optimised to score highly.

 

Paint at its simplest consists of a pigment suspended in a resin. The pigment is solid particles and the resin starts off liquid and solidifies around these as the film dries/hardens. Therefore, unlike a dry stone wall where the space around the stones is a void, in paint the space is filled with a resin. A coffee and walnut cake made with an awful lot of walnuts might be a better analogy, where the walnuts are the pigment and the cake mix is the resin. Once cooked (hardened) if you dunk it in your tea it will soak it up, but that all goes into the sponge part, not the walnut particles. The closer you pack the particles together, ie higher pigment loading, the harder it is for the water to soak through. However, there is a practical limit to how high a loading you can add as above a certain point it won't flow properly, and beyond that there would come a point where there wasn't enough sponge to fill the gaps around the walnuts, at which point it would have holes so would become less effective to add more particles. To a point, you can make it still flow by adding more water, but you then have to evaporate it out again and if the particle loading is high, that gets increasingly difficult so you hit a practical limit of drying time.

 

There are many choices of resin and pigment. Resins solidify either by evaporation of the solvent or by chemical reaction, or by both in turn. Paints which simply dry are usually much less effective, but the resin is cheaper. At the opposite end of the scale you have a 100% solids resin which hardens by chemical curing, very much the same as Araldite epoxy resin (because it is essentially the same thing). All organic materials are permeable to oxygen and water vapour to a greater or lesser extent, but stronger chemical bonds, greater levels of densification and not having any solvent to evaporate out produces a much less permeable film. Add in better bonding to the metal surface so there is no undercutting (where eventually the paint falls off in sheets due to the rust spreading underneath) and you have a much higher performance paint. However, different resins also have different price points and those which chemically cure are mostly two pack so have to be mixed thoroughly in about the right ratio and, once mixed, have a pot life before they cure. This means epoxies are not always the best option where some of the other parameters are more important.

 

The pigments also have several functions. They provide a barrier but they also give colour and can directly add corrosion protection (e.g. zinc phosphate). Many pigments are more expensive than the resin so generally cheap paints have low pigment loadings, making them less effective barriers and also often needing more coats. Inorganic/mineral pigments give very good colour stability but many are particularly expensive and some are toxic. The exceptions are carbon black and iron oxide (rust) which are cheap but a poor barrier. The main pigment is titanium dioxide which is stable and a good barrier and white. It also has very high colour opacity so fewer, thinner coats are required for colour coverage. Titanium dioxide is white so colour is then added via the resin instead, but that is organic colour and hence less stable so gradually fades. Zinc pigments are more expensive so are not added to cheap paint but are to expensive, better quality paint.

 

Alec

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.