Corrosion Inhibitors: Types & Selection Tips for Coating Formulation

Substrate Corrosion & its Mechanism

While the coating itself plays an important role in the corrosion protection, using liquid corrosion inhibitors helps and improves considerably this property.

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

Reduce the amount of anti-corrosion pigments
Provide excellent results & an alternative to cost reduction and environmental questions.

But, before heading towards the inhibitors of corrosion, 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”.

The conductivity of the 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
Damage the adjacent parts

Apart from color and appearance change, it can weaken the structure/destroy the structure itself.

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 the same metal system, where differences of potential exist 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.

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

Surface quality: Heterogeneous surface will increase the risk of corrosion. Treated surface will prevent it. Before the coating application, the surface must be free of contamination.

Adhesion of the coating layer: Coating will form a protective barrier on the metal surface. Lack of adhesion will be weak points with a high risk of corrosion development. A perfect surface wetting is required...

Quality of the coating layer: Pinholes, craters and other surface defects will also weaken the metal protection.

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

Application - Flash rusting with waterborne paint

The waterborne paint is applied directly to the metal. Appears shortly after the application, caused by the migration of rust through the film.
Also, application of the paint on a rust contaminated substrate can be a source of corrosion.

Paint and substrate aging - Long-term Corrosion

Aggressive environment, pollution, weathering can weaken the paint film and increase the risk of corrosion development.
The paint protective barrier deteriorates and weak points appear. 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 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, like corrosion inhibitor zinc phosphate. But some of these pigments tend to be environmentally 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.

Understand what is organic corrosion inhibitors working mechanism from the figure below:

How Corrosion Inhibitors 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 control inhibitor can make a complex with a potential corrosive component and neutralize the corrosion reaction.

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

Risk of Corrosion		Solventborne	Waterborne
In-can Storage	Vapor phase	★	★ ★ ★
In-can Storage	Wet phase	-	★ ★ ★
Application Flash Rusting		-	★ ★ ★
Long-term Corrosion		★ ★ ★	★ ★ ★

Solution Against the Corrosion		Solventborne	Waterborne
In-can Storage	Vapor phase	-	Flash rust inhibitors
In-can Storage	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

Substrate Pre-treatment for Anti-corrosion Benefits

When coatings are used as the means of reducing corrosion, it is essential that the coating adheres very tightly to the surface. For maximum adhesion, the substrate must be prepared correctly. Different methods/types include:

Conversion Coatings

The conversion coating acts as an excellent base for paints and at the same time provides excellent corrosion protection. A conversion coating is a slightly acidic aqueous solution (water-based) of chemicals. Iron or zinc phosphates are the most common chemicals in the formulation, although other chemical salts are also added to perform various functions. The metal is usually immersed in a tank containing the solution. While immersing the metal dissolves very slightly and the phosphate actually plates out onto the clean metal.

Wash Primers

Wash primers are applied to surface prior coating:

To passivate the surface and temporarily provide corrosion resistance
To provide an adhesive base for the next coating

Electrocoating (OEM process)

Electrocoating employs an electric current to deposit an organic finishing process that uniformly applies thin-film primers and one-coat finishes to metallic substrates.

Four steps are involved in the electrocoating process:

Substrate cleaning
Conversion coating
Sealing
Drying, and cooling

Main Primers

Primers are used to 'seal' the surface so that the solvents or water of top coats will be able to evaporate away as they were designed to barrier to oxygen, moisture and corrosive compounds at the metal surface.

Corrosion Inhibitors Selection Criteria for your Coating Formulation

Flash Rust Inhibitors

Waterborne coatings are more sensitive to the corrosion, as they involved… water. Also, many metal cations (such as Fe²⁺, Iron II) are soluble in water. Flash rusting, a fast corrosion development which appears only with waterborne coatings applied directly on metal, when the paint layer is still wet, is a typical example.

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 corrosion inhibitors). 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 corrosion protection by inhibitors. Liquid corrosion inhibitors work in synergy with the anti-corrosion pigments.

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 inhibitors of corrosion:

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

For long-term corrosion inhibitor selection, 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 anti-corrosive pigments
The global cost formulation
The environmental, health and safety restrictions

Also, there are variety of pigments available with their anti-corrosive benefits. Let's have a look:

Barrier Pigments

Many pigments operate mainly by offering "passive protection", enhancing the barrier effect of the coating. Mica, aluminum, glass flake and micaceous iron oxide (MIO) are all widely used. Their effectiveness depends on the fact that they have a lamellar, flake form and will normally align themselves more or less parallel to the surface of the coating. This reduces water and ionic permeability by forcing ions or water molecules to take an indirect path from surface to substrate as shown in the figure below:

Lamellar Extender for Corrosion Protection

Lamellar extender (Top) showing barrier effect reducing moisture penetration,
with near spherical particles (below) for comparison

Talc, usually classed as an "extender" rather than a primary pigment, is also commonly found in anticorrosive paints because it is both highly inert and has a lamellar form.
MIO is a highly effective anticorrosive pigment. There is currently a trend to blend this material with non-lamellar MIO which can be readily obtained for a tenth of the price.
Of the other common barrier pigments, mica is almost totally inert. Aluminum flake is sensitive to moisture and alkaline conditions. Stainless steel flake finds some applications, but is relatively expensive. Glass flake is popular in high-build coatings for heavy-duty applications.

Anti-corrosive Benefits of Barrier Pigments

Active Protection Pigments

Zinc phosphate has established a strong position as an active pigment in anticorrosive primers. It is considered to have three protective mechanisms:

Formation of a protective anodic film
Phosphate ion donation to the substrate
Formation of anticorrosive complexes with certain binders

Zinc phosphate modifications include, for example: Aluminium zinc phosphate, Zinc molybdate phosphate and Zinc silicophosphate hydrate.

Calcium modified silica gels represent eco friendly corrosion inhibitors. The pigment is heavy-metal free, non-toxic, micronized, amorphous particles that offer an alternative to government non-compliant, anti-corrosive agents. Calcium modified silica gel is slightly alkaline (pH 9-10) and is manufactured via an ion-exchange reaction at the surface of silica gel between weakly acidic silanol groups and calcium hydroxide. Calcium modified silica gel is a porous solid having a low density and high surface area compared to heavy metal anti-corrosive pigments.

Therefore, the amount of calcium modified silica gel required to provide anti-corrosive protection is significantly less compared to heavy metal containing anti-corrosive agents. Calcium modified silica gel protects metal surfaces through a mechanism by which calcium ions and soluble silica species diffuse. And develop cathode and anode sites and suppress the corrosion process. Calcium modified silica gel is generally used for coil coatings and thin-film applications.

Calcium strontium phosphosilicate is a relatively new zinc-free anti-corrosive pigment and is considered more environmentally friendly than corrosion inhibitor zinc phosphate. Treating the surface of calcium strontium phosphosilicate with specially designed organic compounds improves the wetting and compatibility with different coating compositions. Moreover, calcium strontium phosphosilicate can also be used in a wide variety of water and solvent-based coating systems.

Aluminum phosphate used as an anti-corrosive pigment is aluminum tripolyphosphate (Al₅P₃O₁₀). Aluminum tripolyphosphate is considered to be an environmental friendly pigment and has been available for use as a low cost anti-corrosive pigment since the mid-1980s. Aluminum tripolyphosphate can be used in a wide variety of solvent-based coating systems as well as water-based coatings. It also has been found to be useful in heat resistant coatings.

Permutations and Combinations

A number of elements and compounds may be considered to exert some protective effect against corrosion. And this has led to the evolution of a wide range of pigments which turn out, on examination, to feature the same relatively small range of protective materials in different combinations. Some further examples (necessarily incomplete) may be briefly mentioned:

Molybdates are effective but expensive, and so usually found in the form of compounds that incorporate other anti-corrosive elements such as zinc molybdate, calcium zinc molybdate and zinc molybdate phosphate.

Aluminum tripolyphosphate (also available in forms modified with zinc ions or silicate) - the tripolyphosphate ion is able to chelate iron ions, in addition to the protective effect of the phosphate itself.

Silicates may be found in the form of combinations such as calcium borosilicate, calcium barium phosphosilicate, calcium strontium zinc phosphosilicate, strontium phosphosilicate, barium phosphosilicate.

An oxyaminophosphate salt of magnesium is offered commercially, though it is recommended only for use in solvent-borne primers. With a relatively low specific gravity of 2.2, it can be used at a lower weight addition than zinc-based pigments.

Conductive Polymers

Inherently conductive polymers, of which the most widely known is polyaniline, are a truly modern development. And among their many applications they have been found to have a dual anti-corrosive effect:

A catalytic reaction with steel produces a thin, dense layer of Fe₂O₃ oxide, which has a barrier effect similar to that of the Al₂O₃ layer that forms naturally on aluminum
A cathodic protection mechanism which is similar to that offered by corrosion inhibitor zinc

Thus, the polyaniline must be in direct contact with the metallic substrate in order to be effective. It has been shown to work well as a thin film pre-treatment under other anti-corrosive paints, and has been commercialized in the form of primers. These primers are claimed to outperform zinc-rich primers when overcoated with epoxies, being able to protect the surface even when coating damage extends to a 2mm wide scratch.

It has been further claimed in a patent that this level of protection can be enhanced by incorporating sacrificial anodic metal or metal alloy particles along with inherently conductive polymers such as polyaniline. In this way both barrier and anodic protection systems are applied in a single coating.

Testing the Inhibitors of Corrosion

For better results, different inhibitors of corrosion used at different dosages should be tested. Generally, up to 3.0 – 4.0% on total formulation. Of course, the paint stability and properties must not be altered by the use of this corrosion inhibitor.

Liquid corrosion inhibitors work in synergy with the anti-corrosion 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 Tests

QUV Cyclic Tests
- QUV Condensation (ASTM G154)
  - Cycle-UV light* -4hr followed by condensation cycle-4hr
  - Condensation Cycle-chamber maintains 100% RH, 50°C *Fluorescent UV lamps
- QUV Prohesion (ASTM G85 A5)
  - Cyclic, panels exposure to wet/dry periods
  - Cyclic corrosion test consisting of one week in QUV and one week in prohesion cycle*
  - UV exposure

Xenon Arc Exposure (ASTM D2568, G26)
Simulates full spectrum solar radiation-UV, visible, and infrared.

Static Tests

Salt-spray Test (ASTM B-117)
A 5% sodium chloride solution is sprayed by means of a nozzle into a closed chamber to produce a static fog. The panels are suspended in this for a prescribed period of time. The temperature is kept constant (95°F). Poor correlation exists with the expected life of a coating.

Controlled humidity test (ASTM D2247)
Estimates the influence of moisture on corrosion. Samples are exposed to 100% relative humidity.

Immersion Test (ASTM D870)
Samples are immersed in 100°F de-ionized water bath.

Electrochemical Impedance Spectroscopy (EIS)
A small amplitude signal is applied to a previously immersed paint panel over a range of frequencies. EIS measures the breakdown of a coating due to electrolyte attack. Estimation of corrosion rates (30 min to 24 hr after immersion) is rapid.

Filiform Corrosion Test (ASTM D2803)
Scribed panels placed in corrosive atmosphere (salt spray for 4 to 24 hours) or immersed in a salt solution Panels exposed to humidity (77oF & 85% RH)

More Testing Methods

Exterior exposure
Humidity test (ASTM D2247)

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.

Anti-corrosive Benefits of Organic-inorganic Hybrid Coatings

The term "hybrid coatings" is rightly used in connection with many different systems in which two (or more) binder systems with distinct properties and curing mechanisms are present. While there are many such systems, the greatest potential for increasing levels of coating performance - or achieving effects which cannot be obtained in any other way - lies with the extreme case of a hybrid coating. That in which organic and inorganic components are combined at a molecular level or at the level of fine functionalized nanoparticles.

Some of the most commonly used hybrid coatings includes:

Zinc-rich silicate coatings
Epoxy siloxane hybrid coatings
Sol-gel coating

Zinc-rich Coatings - The classical example is that of zinc-rich silicate coatings containing small amounts of organic binder materials (and in particular alkyl silicate types). This form of hybrid coating has been used to give outstanding corrosion protection.

Organosiloxane heavy duty coatings - Epoxy siloxane hybrid coatings have been found to provide better exterior durability than even two-pack polyurethane coatings. The binders can be formulated to give a very low viscosity, allowing coatings to be applied at VOC levels of around 120 g/l and film thicknesses of up to 200µm. They are also highly resistant to graffiti, inert against most nuclear radiation, fire-retardant and corrosion resistant.

Sol-gel Coatings - Urethane modified polysiloxane sol-gel coatings have excellent adhesion to metals such as aluminum. Also, they resist chemical attack effectively due to the formation of a tightly packed crosslinked network. They have been found useful as protective coatings on items such as heat exchangers, which have an array of tightly packed metal 'fins' which are difficult to coat, and for which low film builds are desirable.

Corrosion Inhibitors for Paints & Coatings

View a wide range of corrosion inhibitors available in the market today, analyze technical data of each product, get technical assistance or request samples.

Suppliers

Brands

Corrosion Inhibitors for Coatings: Main Types & Selection Tips