|
|
|
 |
Letter 1341 Adhesion problems on hot dip galvanized articles--- I run a powder coating business in which we coat various types of substrates. Our prep systems consists of an alkaline cleaner, rinse, sulphuric acid pickling, rinse, iron phosphate, rinse and a final acidified rinse (chromated). On the odd occasion we need to powder coat hot-dip galvanized articles, but for some reason the adhesion on these articles are very poor. The articles are checked for water breaks after the alkaline cleaning stage, rinsed for a minute or two and the left in the phosphate solution for 7 to 10 minutes. Thereafter rinsed in water and passivated before powder coated.
Thank you for your response. Laurence van Niekerk
--- Galvanizing means applying a hot-dipped zinc coating, often finished off with a chromate conversion coating. It is possible to paint a galvanized surface, but it is different than painting a steel surface. This response has come up so many times that we added it to our FAQs, but: there is a booklet entitled "Painting Galvanized Steel Structures" that has all the information you seek. Single copies are available free from the AGA at http://www.galvanizeit.org/publications/publications.htm
--- Laurence: The chemistry of an iron phosphate on a zinc surface is simple. It can deposit a nice zinc phosphate coating under the right conditions or it can do nothing at all to the surface. Everything depends on the pH of the phosphate solution. If the pH is on the high end say 4.5 to 6.0 then it cannot pickle the surface so it will not deposit anything. If you lower the pH to 3.0 to 4.0, then you begin to pickle off some of the zinc. The zinc stays local to the surface and redeposits on the surface as zinc phosphate. You do not need to change the bath to a zinc phosphate bath, because an iron phosphate bath done right, pulls the zinc out of the parts and puts it right back on as a zinc phos coating. For improved adhesion on the zinc try adding a little fluoride 400-600 ppm to the phosphate bath. This gives a little bit of extra bite into the metal. Hope this was helpful.
I wasn't going to be repetitive since this is covered in detail in the referenced AGA booklet, but here's a common problem -- Just as zinc plating is almost always chromated, galvanized coatings that are not specifically intended to be subsequently painted are also usually chromated. Chromate is one of the best passivation/separation films ever invented: extensively used on stainless steel sheets in electrorecovery cells so that the plated coatings will peel right off; used on all sorts of electroforms for making CD's, vinyl records, and other precision stampers, so that the two surfaces will separate without a fight. If you are intending to paint a galvanized coating you need to specify that it is not chromated. There are simple spot tests for revealing chromate coatings, and I suspect that you will find that the material was chromated.
--- I suggest to identify the adhesion problem between phosphate coating and powder coating or between steel substrate and the phosphate coating. If the former is a problem, I guess the phosphated sheets were contaminated with oil or dust in waiting room or the powder coating was improperly cured. It has to be noted that phosphate coating consists of inorganic salts, the thermal properties of which are definitely different from those of organic powder coatings. If the latter is a problem, there is something wrong with the phosphating process, such as surface cleaning, pH value, temperature, agitation, and concentrations. Adding some wetting agents is helpful to improve the adhesion of phosphate coating on steel substrate. Ling hao
Craig / Ted/ et al Thanks a million for your responses. Firstly I reduced the bath pH to 3.5 and observed a dull gray colour on the galvanized surface. (Would assume this must be the zinc phosphate or an etched surface). Thereafter I increased the pH to above 4.5 to evaluate adhesion properties. Unfortunately I could detect no improvement with either procedure. Ted, I have applied for a copy of the AGA booklet, but as yet have not received anything. Unfortunately I can not close shop until I receive this booklet, so I must persist in my questions. I contacted the supplier of the hot dip galvanized items and he confirmed that the items were chromated. It is however difficult to control this aspect of incoming work as we have various customers who require powder coating of galvanized items.
More questions:
Thank you Laurence van Niekerk
Laurence: I suspect that your experiment with the pH was thwarted by the Chromate.Here are my answers to your further questions. Yes you can remove the chromate in Nitric acid, but you'll have a bath full of zinc and chrome. If adhesion is acceptable to your customers a clean surface is all that you should need to paint over. Zinc phosphate is a misnomer. the crystal contains both iron and zinc. The ferrous phosphate basically becomes a sludge. Unknown on the chromate conversion coating. I would review what adhesion requirements that the customers have and run sample lots to determine if the current system meets requirements and compare that to just an alkaline clean and rinse before painting. Good Luck!
--- Hi Laurence, It depends on how desperate you are. Time saving (abrading) and grit blasting have been used under circumstances such as yours. Just trying to help. R.Sivakumar
--- Dear Mr Niekerk, We have an alternative method of Cleaning/Pickling of metal along with a protective coating, all in one dipping process, in a non-toxic, non-hazardous, non-flammable and water soluble product called "PICKLEX" (does not fall under any EPA or OSHA regulation). PICKLEX prepares the surface for painting, welding, electroplating and powder coating. Try PICKLEX, it may work in your situation. PICKLEX replaces the hazardous/toxic chemicals including phosphate coating, from traditional cleaning/pickling process. Unlike traditional cleaning, Picklex prepares the metal surface with zero rust and keeps it rust free for a very long period of time (in the dipping process there is no washing involved, the metal comes out with a protective coating, not getting any chance to rust). Also, it eliminates hazardous waste disposal, EPA & OSHA problems, and takes lot less time, saving labor, time and cost. Please contact me for more information. Thank you, Ranjit Sen
-- I have problems in degreasing the galvanised sections which I'm having even after degreasing them the " water break " test fails can you give me a good reason why this happens when the degreasing pointage is set correctly there is no oil on the bath the dip timing was also extended Thanks ANIL TIWARI
Can you tell me the proper procedure for painting a galvanize garage door. I painted it last summer and its all peeling. It was primed with Tremclad galvanize primer. Hope you can help me. Thank You. Jacques Perrier
Our company ran into a problem using galvanized, chem-treated sheet steel to make billboard panels. The standard posting paste is water-based and apparently the chem-treatment prevented adhesion. Is there anyway to remove this effect in the stock we have remaining? Jesse Robertson
. To be able to paint hot dip galvanized steel and to do it properly you need at minimum 5 steps.
Do not attempt to use Iron Phosphate unless you just want to use the fluoride (Assuming your Iron Phos Product contains Fluoride) for an etch. If you attempt to develop a crystalline structure using Iron Phosphate you will reapply zinc as a powder (akin to talcum powder). Adhesion will be worse than a simple etch. If you are going to do it, do it right. Tom Roberts
+ I was able to troubleshoot this exact problem on a 5 Stage Iron Phosphate line. The HDG parts had an inconsistent coating evident by different reactions to Copper Sulphate along the surface. They were also over 18 feet long, so a manual pre-clean or blast wasn't feasible. Discussions with the company doing the galvanizing didn't help. The cleaner and phosphate stages had to remain compatible for all regular substrates, so the basic parameters such as pH, concentration, pressure, time etc. could not be permanently changed. The easiest solution was a set of knockdown risers at the entrance using a 5 %(v/v) Sodium Hydroxide-based cleaner formulation. I believe that this is removing any oxide and/or passified areas on the surface. The parts then run through the regular multi-metal cleaner (which has to be tightly monitored for free caustic drag-in). Adhesion is uniform and excellent. We are now experimenting with ramping the regular Potassium Hydroxide-based cleaner with NaOH just before a run, and then buffering it with Phosphoric Acid after. The Phosphoric will produce Trisodium Phosphate [link is to product info at Amazon], which is a half-decent multi-metal cleaner itself! Hope this helps. Steve Cox
Dear Reader: please choose what you want to do.
 |
|
Save
This Page (why?) - Home - ©1995-2008 finishing.com
