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Corrosion Inhibitors for Coatings

Corrosion directly impacts the aesthetics and quality of many finished goods. Apprise yourself with the role that coatings play in the protection of metallic surfaces against the corrosion.

Understand in detail, when does corrosion occurs and thus explore:

Corrosion Inhibitors - Types and Selection


Corrosion Inhibitors for CoatingsWhile coating plays an important role in the corrosion protection, using liquid corrosion inhibitors improves this property.

These agents can be used alone like in clearcoats or in synergy with various anticorrosion pigments. This synergy improves the corrosion resistance of the paint and it is even possible to:

  • Reduce the amount of anticorrosion pigments
  • Provide excellent results & an alternative to cost reduction and environmental questions.

But, before heading towards the corrosion inhibitors, let's understand the phenomenon of corrosion first.


Corrosion in Coatings


Corrosion is an oxidation-reduction reaction in the presence of an electrolyte, leading to deterioration of metal. Typically, for ferrous materials such as iron and steel, corrosion is also called “rust”. Also, conductivity of electrolyte is crucial:

Higher the conductivity, faster the corrosion


That is why, rust develops faster in salt water than in pure water.

The corrosion of a metallic part can change the surface aspect, weaken its properties and damage the adjacent parts. Apart from color and appearance change, it can weaken the structure/destroy the structure itself.

Rust

In coatings, electrochemical corrosion is predominant. It is the combination of two conductors (electrodes) with an aqueous electrolyte solution. The metal with more negative potential will be the anode, and will corrode, where the one with more positive potential will be the cathode. Then occurs an oxido-reduction reaction, in the electrolyte solution.

But corrosion can also occur in same metal system, where differences of potential exists on the surface. These differences of potential can come from heterogeneous chemical composition like:-
  • Differences in the coating layer
  • Contamination
  • Scratches
  • Pinholes…

In iron, corrosion occurs when different parts of the surface creating anode and cathode, are exposed to electrolyte solution. Without any electrolyte, the corrosion is strongly reduced. In other words, salted atmosphere (like marine conditions) are more aggressive than non-polluted atmosphere. In pure water, corrosion is nonexistent.

Single metal corrosion

Besides this corrosion reaction, many other factors may influence the corrosion of the coating, such as :



When Does Corrosion Happen?


The corrosion risk is present all along the coating life, from the storage of the liquid paint itself (in-can corrosion), to the application (flash rusting) and many years later (long-term corrosion):

Storage

In-can corrosion. Critical for waterborne coatings.

In-can corrosion

In-can corrosion 

During the storage, the paint is directly in contact with the iron can, provoking the corrosion.

Application

Flash rusting with waterborne paint. Also, application of the paint on a rust contaminated substrate can be a source of corrosion.

Flash rusting

Flash Rusting

The waterborne paint is applied directly to the metal. Appears shortly after the application, caused by the migration of rust through the film.

Paint and substrate ageing

Aggressive environment, pollution, weathering can weaken the paint film and increase the risk of corrosion development.


Long term corrosion

Ageing – Long Term Corrosion

The paint protective barrier deteriorates and weak points appears. Besides, the no protected substrate parts can be attacked by the corrosion.



Strategies to Control / Reduce Corrosion


The corrosion control involves natural chemical reactions between the metallic substrate and its environment. There are some solutions to control and reduce the corrosion development:
  • Modify the metal properties: Pre-treatments improve the corrosion resistance of the metal.
  • Change to non-metallic materials: But, this cannot fulfill all the final product requirement…
  • Impose an electric current to supply electrons: Expensive and not always realizable!
  • Use a sacrificial anode: A paint formulation rich in protective pigments based on zinc.
  • Use anticorrosive pigments: The most common solution, anticorrosive pigments will chemically passivate the metallic surface in time (especially chromates, phosphates and molybdates) and can act as sacrificial pigments when combined with zinc oxide. But some of these pigments tends to be environmental unfriendly.
  • Use an organic corrosion inhibitor agent: Based on various structures, such as amine, acid, polymers, salts, these products will form a protective barrier on the metal surface and break the chemical reaction, preventing the rust to develop. The passivating layer prevents the metal to be oxidized.
Anodic passivation


How Does a Corrosion Inhibitor Work?



The corrosion inhibitor can form a protective layer at the metal surface by:
  • Chemical adsorption
  • Ionic combination
  • Oxidation of the base metal (especially with Aluminum)

The corrosion inhibitor can make a complex with a potential corrosive component and neutralize the corrosion reaction.

We can summarize the risk of corrosion and how improve the corrosion resistance from the formulation side:

Risk of Corrosion

Solventborne

Waterborne

In-can Storage

Vapor phase

★ ★ ★

Wet phase

-

★ ★ ★

Application
Flash Rusting

-

★ ★ ★

Long term Corrosion

★ ★ ★

★ ★ ★



Solution against the corrosion

Solventborne

Waterborne

In-can Storage

Vapor phase

- Flash rust inhibitors

Wet phase

-

Flash rust inhibitors

Application
Flash Rusting

-

Flash rust inhibitors

Long term Corrosion

Anti-corrosion pigments Corrosion inhibitor agents

Anti-corrosion pigments Corrosion inhibitor agents




Select the Best Corrosion Inhibitor for your Formulation


Flash Rust Inhibitors


Flash rusting is fast corrosion development. It occurs only with waterborne coatings. As they involve water, they are more sensitive to the corrosion. Waterborne coatings applied on metal, when the paint layer is still wet - a typical victim of flash rusting. Waterborne coatings in contact with metal, pose high risk of flash rusting and in-can corrosion. Therefore, it becomes necessary to use a flash rust inhibitor.

Most flash rust inhibitors contain sodium nitrite (toxic). Nitrite free inhibitors are also available. They must be used at higher dosage (up to 1.5% on total formulation).

Eco-friendlier nitrite and borate free versions replace the ones that are water soluble / dispersible and sodium nitrites based . Most of the products in the market have a dosage level between 0.2% to 1.5% (delivery form on total formulation) to have a significant effect on the in-can corrosion and flash rusting.

NOTE:

Calcium based corrosion inhibitor gives better compatibility in waterborne. It can help the pigment dispersion when used at the pigment grinding stage. Although, some emulsion resin can be sensitive to the Ca2+.


Long Term Corrosion Inhibitors / Protectors


Apart from anti-corrosion pigments, liquid organic corrosion inhibitors also provide long term protection & resistance to corrosion. Liquid corrosion inhibitors work in synergy with the anti-corrosion pigments.

 » Explore All Anti-corrosive Pigments Available Today! 

As corrosion reaction is an oxidation – reduction chemical process, the required corrosion inhibitor metal variant can be first selected using the chemical standard reduction potential. This scale is a first approach, as values are based on measurements in aqueous solution at 25°C, which is not the ideal case of all coatings !

Then it becomes easy to select a metal version of the corrosion inhibitor:

  • First selection choice will be a corrosion inhibitor based on Barium.
  • In case of Zinc based anticorrosion pigments: Zinc based corrosion inhibitor
  • In case of new and less toxic anticorrosion pigments: Magnesium based corrosion inhibitor
  • Amine and Polymeric based corrosion inhibitor for a metal free alternative

For long term corrosion resistance, the type and dosage of the agent is influenced by:

  • The type of metal to be protected
  • The protection effectiveness in time under defined condition
  • The presence and effectiveness of anticorrosive pigments
  • The global cost formulation
  • The environmental, health and safety restrictions


Testing the Corrosion Inhibitors


For better results, different corrosion inhibitors used at different dosages should be tested. Of course, the paint stability and properties must not be altered by the use of this corrosion inhibitor.

Generally, up to 3.0 – 4.0% on total formulation


Liquid corrosion inhibitors work in synergy with the anticorrosion pigments. They also improve the long term corrosion resistance.

To offer the best performances, they should be perfectly dispersed:

  • Preferably added during the pigment dispersion stage to ensure a perfect homogenization. In case of post addition, enough stirring is required

  • In waterborne, a premix with a neutralizing amine, and/or the coalescing solvent may be necessary

About the substrate, surface preparation and especially the liquid paint wetting and adhesion are crucial. Contaminated, dirty and porous surface will increase the corrosion sensitivity. Rough surface after sanding will improve the paint adhesion.

To lead laboratory tests, using some
standardized panels for corrosion tests is highly recommended

After formulation and complete curing, the paint should be tested under different corrosion method, such as:

  • Cyclic corrosion test (ASTM D5894)

  • Exterior exposure

  • Humidity test (ASTM D2247)

  • Salt Spray (ASTM B117)

Other corrosion test can be done such as accelerated weathering with specific appliances, or specific spray tests to reproduce the condition of polluted atmospheres.


The objective is achieved when the corrosion is under the limit level after the required time


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