Anticorrosive polyester powder coating for corrosion sensitive substrate

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Francesco Bellucco, Andrea Perelli -  Pulverit SpA, Italy

Abstract. The purpose of this project is to demonstrate the anticorrosive properties of a completely new powder coating based on Saturated Polyester Resin for application Qualicoat Class 1 and Qualicoat Class 2 applied on a corrosion sensitive substrate as the Aluminium alloy 2024.

Introduction

In the economy of an industrialized country, the costs linked to the corrosion damages represent roughly the 6-8% of the gross domestic product. In Europe the estimation of these costs are higher than 750 Billion euro per year.

To counteract this phenomenon several coating system are available, both with liquid and powder paints. We can divide it into two main categories: Zinc and Zinc Free system. The first category is the most popular in the anticorrosive application due to the high anticorrosion resistance. The anticorrosive property is due to the presence of Metallic Zinc, acting as sacrificial anode, preserving the metallic substrate. Despite it is an ancient technology it is still commune. Nowadays the main problem is the Zinc environmental impact; the Zinc is dangerous for aquatic organism and has a long-term effect on them. The environmental impact is not the only defect of the Zinc primer, there are others negative aspect to consider in terms of applications as:

1)      Zinc primers have a high specific weight compared with standard products so the yield of this product is poor.

2)      The ratio between the binder and the Zinc is very low, it means the substrate has to be pretreated or pre coated with a first layer in order to guarantee a good adhesion of the Zinc coating

3)      The durability of the protective film is proportional to the quantity of the metallic Zinc, the best results are obtained with a percentage of 60-70% of free Zinc. In the case of powder coating, we cannot exceed 35-40%, so the performance is limited.

Considering the negative aspects of the Zinc system in the powder coating application, the trend is to move to Zinc free products maintaining the same performances.

In order to achieve this target a possible solution is a double layer system. Normally the chemistry of the two layers is different: the first layer, the one in contact with the metal, is an epoxy system, the second one is typically a polyester or polyurethane in order to add UV resistance to the top coat. The choice to have an epoxy as first layer is due to the fact that this system has a high number of “free –OH” that allow a very good adhesion on the metal substrate creating a dative bond between the complete electronic orbital of Oxygen and the empty electronic orbital of the metal substrate. Moreover, the epoxy chemistry create a high density crosslinking net, making the permeability of the film lower than other systems increasing the anticorrosive properties. This solution is a good compromise between anticorrosive properties and environmental impact but the cost is higher vs a single layer (double coating cost and double application cost.

Picture double layer with thickness indication.

Another possible solution is to add in the formulation inorganic salts, like Zn3(PO4)2. In this case Zinc does not act as sacrificial anode (because it is already oxidized, Zn++) but act as “filler of the protective film”. Due to its shape, the crystal of this salt creates a barrier and decrease the permeability of the film. The low permeability of the film gives, as final result, a higher resistance to corrosion because water molecules take more time to arrive to the metal substrate (water diffusion is slowed down)

 

Zinc Phosphate in its typical flake shape

This solution has a better enviromental impact due to the absence of free Zinc in the formulation. This solution do not offer high performace resistance because it is only physical barrier and not a chemical protection as the system with metallic Zn (redox).

Pulverit, following the driver of enviromental sustanibility, cost saving and high performance of coating film has created a new line of anticorrosive powder coating based on a completely new additives. Endurance serie combine very good outstanding properties with high chemical resistance, mechanical resistance, scratch resistance and anticorrosive properties. This new line of Polyester Powder coating increase its own anticorrosion properties working in three different way:

1)      Create inside the polyester film a phisical barrier using special bi-dimesional filler that is able to orient the flakes in parallel of the substarte creating an impermeable layer that slow down the diffusion of water in the polymer. The diffusion inside the polymeric film is reduced up to 90% in relation to the content of fiber in the polymer.

2)      Change the rehology of the polymer: the content of these special fibers inside the polymer increase the visosity and change the rehology of the film giving the advantage to cover perfectly also the edges of the support. This perfect coverage of all the edges increase a lot the resistance to corrosion because there are no uncovered areas where corrosion can start easyly.

3)        Passivation of the metallic substarte by Aminophosphate of MgO. Working with this particular additives amines create a protective film by absorbtion in the metallic substrate and thanks to PO43- and Mg++ Ions a passivation of metal by Fe3(PO4)2 and Mg3(PO4)2 reduce the kinetics of corrosion reaction. Moreover for each Mg++ Ions that is released 2 OH- ions are released and they contribute to suppres the catodic reaction 2e- + H2O + ½ O2 à2 OH- 

Physical barrier

The special fibers used in the formulation of Endurance serie work in two direction: the first one is to increase the barrier effect decreasing the migration speed of moisture inside the polymer and the second one is to increase the hardness and the anti scratch properties of powder coating film in order to preserve it during the handling phase in order to preserve the surface of metal from accidental damanges that allow the corrosion to start quickly.

 

As indicated, increasing the quantity of fiber into the formulation, increase the resistance to moisture migration and also the surface hardness misured by a Taber Test. Intresting is to underline that the “barrier effect” is obtained with a small amount of fiber inside the formulation and, even if th quantity of fiber increase a lot, the barrier effect stay pratically at the same level (the “plateau” of effect is achieved easyly with a small amount of fiber). On the other hand, concerning mechanical properties, if it’s necessary to achieve a very hard surface with a good Taber resistance it’s necessary to load in formulation a big amount of fiber paying attention to the secondary effect that this quantity will bring as conseguence (orange peel, weakness in adhesion, finer particle size of powder coating due to the higher hardness of the mixture)

Change in rehology of the film

The special fiber used in the formulation of Endurance serie strongly modify the rehology of the binder giving some advantages, in particular:

1)      Pill flow of powder is extremely short. Even if the gel time of the product is up to 300 seconds it’s impossible to note any drop during the pill test flow, this means that during curing cycle it’s impossible to obtain uncovered area. All the powder applied during the application phase melt without flowing away from the initial position, in this way edges and underbody parts (usually the most difficult to cover) are completely covered and no possibility for redox reaction to start.

2)      During the curing process chain conveyors induces vibration on pieces, also in this condition the rehology of melted powder doesn’t change and no flowing from edges or other particular shapes is possible

 

Viscosity of powder@200°

 

Passivation of metallic substrate

Another important topic related to the Endurance series is the auto passivation of the substrate once the coated material interact with moisture of enviromental. Endurance series use a completely different chemistry than the standard anticorrosive , in fact the material included in our powder coating acts in two way: the first one is to create a very thin film on the metallic substrate, this film usually is a monomolecolar layer that is fixed to the metallic substrate trough a dative bond between electrons of Nitrogen and empty orbitals of the metallic substrate. This monomolecular film preserve metal from the oxidation and, in case of oxidation (PO4)3- ions are released creating a passivation of the metal trough the creation of a salt deeply linked to the metallic surface, moreover the chemical mechanism of reaction release MgO that reacts with moisture creating Mg(OH)2. These two OH- ions  suppres the catodic reaction

2e- + H2O + ½ O2 à2 OH-

In fact, according to the Le Chatelier principle, if increases the concentration of ions OH- the reaction will move in the opposite direction, in this way the electrons are not consumed so the electrical circuit is not close and the redox reaction cannot take place preserving the metallic substrate from the corrosion.

Another important difference between Endurance and standard system is the formation of a passivation layer of mixed salts: Zinc phosphate and Magnesium phosphate. This protective layer allows a higher adhesion to the substrate and a higher surface hardness able to prevent the corrosion process. We would like also to evidence the lower electronegativity of Magnesium VS Zinc allow to have more PO43- ions free in the coating film, these ions saturate the metallic ions created on the surface of the metal during the redox reaction. This means less metal oxide on the surface of the metal helping to prevent the beginning of the corrosion reaction, in fact, higher is the quantity of oxide on the surface of metal higher is the porosity of the substrate, faster is the penetration of Oxygen into the surface speeding up the corrosion reaction.

Here we present the result of this theory applied on a HAA polyester powder coating in comparison with a standard HAA polyester applied on a poor Aluminium alloy like the 2024 with three different pre-treatment of the surface:

1- Chrome free (Alugold SCR - 25 g/l - T=25°C - time= 1'30")

2- Chrome III (Surtec 650 - 20 g/l - T=25°C - tempo=1'30")

3- Chrome VI (Covergold 60R - 7 g/l - T=25°C - tempo=1'30")