Metal corrosion is a chemical reaction between a metal surface and its environment that can be induced by many different mechanisms:

• GASEOUS CORROSION

• CHEMICAL CORROSION

• ELECTRO-CHEMICAL CORROSION

• ATMOSPHERIC CORROSION

• CORROSION IN WATER

GASEOUS CORROSION

This occurs at high temperature when a metallic surface is in contact with a dry gas. In most cases it involves the direct reaction of metal and oxygen:

Fe + ½ O2 → FeO ( Iron monoxide)

When iron is exposed to air, redox reactions form iron(III) ions The electrons released reduce the oxygen to oxide ions:

2 Fe → 2 Fe3+ + 6 e− 1.5 O2 + 6 e- → 3 O2- ↓

Fe2O3(Iron oxide)

These are the basics for iron oxide pigments production

CHEMICAL CORROSION

Chemical corrosion involves creation of electric current. Products of corrosion are formed directly:

Fe + H2SO4 → FeSO4 + H2 Ferrous sulphate+ Hydrogen

In non electrolytic environment (gasoline, kerosene, naphtha), corrosive products depends on the presence of additives such as Lead derivatives.

ELECTRO-CHEMICAL CORROSION

The combination of two conductors (electrodes) with an electrolyte is called Galvanic cell, that converts chemical energy in electrical energy.

• Electrode where reduction process happens is cathode

• Electrode where oxidation process happens is anode

That is the reason why corrosion phenomenon (oxidation) appears on anode.Electrons from the corroding anode metal move to the connected cathode where they recombine with oxygen and water in the electrolyte to make a new hydroxyl ion (OH-) Once corrosion starts it will continue until all the ingredients are used.

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BASIC PRINCIPLES OF CORROSION

FACTORS INFLUENCING GALVANIC CORROSION

The most important factors that influence metal corrosion are:

• Intensity of electrons flow

• Surface area of cathode and anode

• Aeration effects

• Stagnation effects

INTENSITY OF ELECTRON FLOW

The intensity of electron flow is dependent on the potential difference (voltage) which exists between the metals.

Potential in volt Elements1.68 Gold

1.20 Platinum

0.799 Silver

0.798 Mercury

0.345 Copper

0.000 Hydrogen

- 0.126 Lead

- 0.136 Tin

- 0.152 Nichel

- 0.227 Cobalt- 0.40 Cadmium

- 0.44 Iron

- 0.71 Chrome

- 0.762 Zinc

- 1.05 Manganese

- 1.67 Aluminium

- 2.34 Magnesium

- 2.90 Barium

- 2.92 Potassium

- 3.02 Lithium

If two metals with a large potential difference are joining between them, it produces higher corrosion rates than if the metals were close in electrical potential.

GALVANIC SERIES OF METALS AND ALLOYS IN SEA WATER

Noble (protected metal) cathodic

• Platinum • Titanium • Austenitic stainless (with

protective film) • Chromium steel s (11-30% Cr) • Silver • Inconel (80% Ni, 13% Cr, 6.5%

Fe)• Nickel and High Nickel alloys • Monel (67% Ni, 30% Cu) • Bronze, Gunmetal • Copper • Alpha Brass (70% Cu, 30% Zn) • Naval Brass (60% Cu, 40% Zn) • Austenitic Stainless (protective

film removed) • High Nickel cast Iron • Chromium Steels • Cast Iron • Mild Steel • Alluminium Alloys

Base (corroded metal) anodic

SURFACE AREA OF CATHODE AND ANODE

The size of the cathode relative to the anode is important:

A small anode connected to a big cathode is forced to supply a huge amount of electrons and will quickly corrode.

A large anode connected to a small cathode can provide easier electrons and will take a long time to show evidence of corrosion.

Less noble metals have to have at least 100 times more surface area than the noble metal

AERATION EFFECTS

The corrosion reaction requires oxygen:

• where oxygen is present the metal is cathodic

• Where oxygen is depleted the metal is anodic and corrodes.

steel posts dug into the ground will rust just below the surface because of this effect

STAGNATION EFFECTS

During corrosion, ions build up immediately around the anode and cathode saturating their respective regions. The corrosion begins to fall due to the concentration of stagnant ions blocking the creation of more ions in the electrolyte. If the ions are removed or more voltage is provided the corrosion rate again picks up.

ATMOSPHERIC CORROSION

Main factor, responsible for the atmospheric corrosion is humidity. In fact:

• Steel abandoned in a desert maintains for long time the characteristic metal brightness.

• In glacial atmosphere corrosion is imperceptible.

At temperature below freezing point, there is no water necessary to for the corrosive process. Ice is a very bad electrical conductor.

STEEL OXIDATION

ATMOSPHERE TYPES

• Low humidity till 50% R.U Corrosive products (oxides) provides a surface barrier

• Average humidity over 50% R.U Corrosion occurs through condensation of water in surface defects or pores developing electro-chemical corrosion.

Corrosion is localized.

• Saturated atmosphere 100% R.U corrosion mechanism is the same as the one of corrosion in water.

Corrosion is severe.

CORROSIVITY OF THE ATMOSPHERE

Many factors influence negatively the atmosphere corrosivity:

• Powders • Gas • Salts

In natural atmosphere, not contaminated by specifically corrosive substances basic factor of corrosion is humidity.

Atmosphere character

Relativecorrosion %

Arctic 1-3

Rural 3-9

Urban 15

Tropical 25

Marine (pure) 38

Industrial 50

Marine (industrial) 65

Industrial (contaminated)

100

CORROSION IN WATER

Since corrosion is essentially a wet oxidation, in a range of temperatures from 0°C to 100°C water is absolutely necessary for the corrosive process.

• Pure water is not too aggressive. • Presence of salts provides the

electrolyte causing corrosion.

Sea water is one of the highest corrosion environments for its high salts content.

SPECIFIC TYPES OF CORROSION

Corrosion produces physical evidence of its presence. The form it takes depends on the mechanism of the corrosion:

– Pitting corrosion

– Crevice corrosion

– Stress corrosion

– Bacterial corrosion

– Galvanic corrosion

PITTING CORROSION

A metal can corrode without being in contact with another metal

In pitting corrosion the metal at the top of the pit has access to the oxygen in the air and becomes the cathode. At the bottom of the pit oxygen is depleted and the metal becomes the anode. The deeper the pit is the higher is the corrosion rate.

CREVICE CORROSION

A crevice is created whenever two objects are brought together. Unless they are perfectly flat a crevice is present and oxygen cannot easily enter the gap but is plentiful outside. Corrosion starts in the crevice because of differential aeration.

STRESS CORROSION

Metal under tensile stress can corrode at higher rates than normally expected. The stressed areas have changed electrical potential to the neighbouring metal and are also more likely to develop microscopic surface cracks. Both situations promote increased corrosion rates.

BACTERIAL CORROSION

There exist many species of bacteria living in the moist environments that release acidic waste products or that can strip out elemental components of a metal:

Acidothiobaccillus is an aerobical bacteria that produce sulfuric acid.

Desulfovibrio and Desulfotomaculum are anaerobic bacterias producing hydrogen sulfide.

Anaerobic corrosion is evident as layer of metal sulfides and hydrogen sulfide smell.

GALVANIC CORROSION

Dissimilar metals of different potentials joined together by an electrolyte, like process water or rainwater, will cause the more anodic metal to corrode. Running copper water pipe to a galvanised tank will cause the tank to corrode very quickly. Joining copper to steel is even bad. In the Galvanic series only join metals that are near each other.

CORROSION SUPPRESSION

Corrosion control involves hindering the natural chemical reactions that occur between the metal and its environment:

• Modify the properties of a metal • Impose an electric current to

supply electrons • Change to non metallic materials

Frequently these systems can’t be used for technical or economical reasons so, more frequently following protection methods are used:

• Modify the environment • Install a protective coating over

the metals

MODIFY THE ENVIRONMENT

The use of a corrosion inhibitor that combines with the corroding metal(anode) or the protected metal(cathode), forms a barrier layer that reduces the flow of ions and electrons across it and virtually stops the corrosion. This is a common technique used in boilers to protect them from corrosion.

INSTALL A PROTECTIVE COATING OVER THE METALS

Metals can be protected by covering them with metallic or non metallic coatings:

- Metallic coatings of less noble material over more noble metal provide sacrificial protection.

- Non metallic coatings can:

• Act as a physical barrier minimizing the permeation of water and oxygen.

• Introduce a very high resistance into corrosion cell circuit through the presence of inhibiting agents.

CORROSION INHIBITORS

A corrosion inhibitor can be defined as a material that reduces the aggressive nature of the corrosive medium by reaction at the metal-to-metal solution or metal-to-coating interface. There is a preponderance of charges covering the metal-to-corrosive medium interface, which is known as the electric double layer. First layer of charges at the metal surface is caused by an excess or deficiency of electrons. Another layer is formed on the solution side of the interface by spatially absorbed ions.

The sandwich of these charges form the inner Helmholtz plane of the double layer

POTENTIAL DIFFERENCE

Formation of an electrical double layer results in a potential difference known as the electrode potential between the surface of the metal in question and the corroding liquid. A metal in contact with a solution containing its ions establishes a potential difference relative to every other metals in the same condition. The introduction of any material into the electric double layer change its composition and structure.

LINEAR SWEEP VOLTAMMETRY (LSV)

Measuring the capacitance of a double layer, before and after the addition of inhibitor, can be used to monitor the absorption of a corrosion inhibitor.

CORROSION INHIBITOR PROPERTIES

Ideal corrosion inhibitor should:

• Be effective over a wide PH range

• React with the metal surface to produce a product with lowersolubility than the unreacted inhibitor.

• Have sufficient solubility to maintain a reservoir on the coating

• Form a film on the coating substrate interface that does not reduce the coating adhesion

• Be effective as both an anodic and a cathodic inhibitor (cathodic polarization can cause loss of coating adhesion).

TYPES OF INHIBITORS

Corrosion inhibitors can be classified as:

• Inorganic inhibitors

• Organic inhibitors

• Barrier pigments

• Sacrificial pigments

SACRIFICIAL PIGMENTS

Sacrificial pigments such as zinc, function initially by cathodic protection of steel and subsequently by the sealing action of the corrosion products on the coating surface. Generally 85-95% of zinc powder based on solid binder is required to provide the necessary electrical conductivity to establish an electrochemical cell making the steel cathodic as sacrificial zinc anode coatings.

BARRIER PIGMENTS

“Lamellar” pigments such as mica and micaceous iron oxides, form a wall of flat, thin particles within a paint film.

These resist penetration, forcing water to wind a long path toward the substrate. Metallic flakes of aluminium, bronze or steel produce similar effects. Since not all pigments are compatible with all resins, care must be taken when adding pigments to coatings. In high

acid or alkaline environments, inert or chemically resistant pigments should be used.

INORGANIC INHIBITORS

Most known inorganic inhibitors are pigments belonging to the following families:

• Lead pigments • Chromates • Molybdates • Borates • Silicates • Phosphates

Each one presenting advantages and disadvantages

The organic corrosion inhibitors provide an alternative to toxic heavy metals for long term and short term corrosion protection and can work synergistically with non heavy metal based anti-corrosive pigments Main advantages of organic corrosion inhibitors are:

• Liquid, easy to disperse • Low dosage compared to

anticorrosion pigments. From 1% to 4% instead of 8% to 20%

• Can be used in varnishes • No negative effect on gloss

Most known organic inhibitors are:

• Basic sulfonates

• Dinonyl Naphtalene Sulfonates

• Amine and amine salts

BASIC SULFONATES

They provide anticorrosion in two ways:

• Having better affinity with metal surface than water, they displace water from the interface preventing corrosion.

• Forms a thin film of Calcium oxide which passivates the surface protecting it from corrosion.

Over based alkylaryl calcium sulfonates are widely used as additives for lubricants. Their use in paints is limited for:

• Their poor compatibility (petroleum oil content)

• Too high level of addition (> 20% on binder solids)

• Adhesion problems

AMINE AND AMINE SALTS

Amine salts of carboxylic acids adsorb strongly on iron surfaces. Electro-sorption of cations and anions of the salt take place because of the low energy of activation. Then chemisorptive bonds are formed, which provide inhibition through a monomolecular barrier.

DINONYL NAPHTALENE SULFONATES

The dinonyl naphthalene sulfonates, are strongly adsorbed on iron oxide powder. The related salts of the acid have an unusual umbrella-like shape with molecular areas of around 200 angstroms.

The result is that the polar groups of the molecule interact with the active sites on the metal oxide surface passivating the induced corrosion cell. The oleophilic structure of the dinonyl naphthalene portion of the molecule orients itself away from the metal surface forming a protective hydrophobic barrier. As a consequence, this sulfonate monolayer significantly reduces the permeation of oxygen and humidity of the coatings, in addition, the high molecular area makes it possible toobtain the requested protection by the use of very small additive levels

FLASH-RUST

According to the common definition flash rust can occur:

• On metal substrates after the cleaning with water and detergent.

• On metal substrate during the application of a waterborne paint.

• In can during paint storage in the area of substrate in direct contact with the paint (wet zone).

• In can during storage in the area of substrate in contact with the water vapour (condensing zone).

FLASH-RUST ON METAL SUBSTRATE DURING PAINT DRYING

Flash rust, is an oxidation of the steel that occurs, within minutes, as the water is drying. Flash rust is also the rusting of steel as a water-borne coating is drying. Steel naturally oxidizes when water is present. Flash rust quickly changesappearance to a rust bloom or water bloom over a large surface area. The color of the flash rust may vary depending on the age and composition of the steel and the time of wetness of the substrate prior to drying.

Without anti With anti flash rust flash rust

In can corrosion occurs on the substrate in direct contact with the paint (wet zone).

Tinplate cans used to pack waterborne paints are usually protected inside with an epoxy-phenolic coating to prevent the aggression of tinplate by corrosive salts contained in the paint. Unfortunately mechanical stress (welding, rims, handling) can damage in some areas the continuity of the paint and consequently create some potential points of corrosion.

Therefore all the paint producers include in their formulation an anti flash-rust additive to avoid in can corrosion phenomenon.

In can corrosion protected with anti flash-rust

substrate in contact with the water vapour (condensing zone)

For samples or more information:

Synthron - 6, rue Barbès – B.P.177 – 92305 LEVALLOIS-PARIS cedex (France) Tel.: + 33 (0) 141341400 – Fax + 33 (0) 141341416

E-mail: postmaster@protex-international.com

Metal corrosion costs industry billions of Euros in the form of loss of infrastructure (e.g. pipework, metal structures) and leakages.

For this reason the protection of metal (especially steel) against corrosion is a very important topic of study.

The main factor responsible for this corrosion is humidity. This fact is confirmed by the long life of steel in deserts or in glacial atmospheres.

The overall reaction forming ferric oxide characteristic red dust) is:

Fe + ½ H2O + ¾ O2 ½ Fe2O3.H2O

Since corrosion is essentially a wet oxidation process, restricting the diffusion of water and oxygen to the steel surface can, theoretically, eliminate this chemical reaction. For this reason, thick coatings (or coating barriers) are used to significantly reduce water vapour diffusion to the interface. Unfortunately even the best systems do not completely prevent the permeation of both oxygen and water to the metal surface. In fact, atmospheric humidity is not a pure liquid. It contains dissolved hygroscopic salts such as chlorides, sulphates and sodium salts. These can penetrate the coating barrier thus drawing moisture to the coating/metal interface and providing the electrolytes needed for the corrosion process.

In view of the considerations above, there has always been comprehensive work carried out on the introduction into the coatings of substances which are able to inhibit corrosion at the interface. In the past, a wide variety of pigments (based on lead, chrome, strontium, barium or more recently zinc) were successfully used. Due to environmental pressures these anticorrosion agents are strongly penalized in terms of labelling.

In alternative solution to toxic heavy metal pigments are the organic corrosion inhibitors such as the DNNS (Dinonyl naphthalene sulfonic acid) salts.

Synthro®-cor liquid corrosion inhibitorsThe Synthro-cor range is amine or metal salt of DNNS acid, suitable to be used to impart corrosion inhibition in aqueous and solvent based coatings.

The dinonyl naphthalene sulfonates, having a heat of adsorption of 10.1 Kcal/mol, are strongly adsorbed on iron oxide powder. The related salts of the acid have an unusual umbrella-like shape with molecular areas of around 200 angstroms. The result is that the polar groups of the molecule interact with the active sites on the metal oxide surface passivating the induced corrosion cell, while, the oleophilic structure of the dinonyl naphthalene portion of the molecule orients itself away from the metal surface forming a protective barrier. As a consequence, this sulfonate monolayer significantly reduces the permeation of oxygen and humidity of the coatings, in addition, the high molecular area makes it possible to obtain the requested protection by the use of very small additive levels.

From an environmental point of view, amine salts are completely acceptable. Also the salts formed from heavy metals such as Zinc or Barium have no significant negative impact, because the metal content is very low; well below the limit imposed by the toxicity labelling. The addition rate of 1 to 3% on total formulation is also very low too if compared with the 5 to 20% or more necessary for classic anticorrosive pigments.

SYNTHRO®-COR Series Environmentally friendly

Anti-flash-rust and liquid corrosion inhibitors

YOUR ADVANCED SOLUTIONS PARTNER

Synthro®-cor selection table

Calcium, Magnesium and Zinc salts can be supplied also in Mineral Spirits

ADVANTAGES

Valid alternative for anti-corrosive pigments that are environmentally unacceptable.

Protection in clear coats and pigmented systems.

Synergistic action with non-heavy metal based anti-corrosive pigments.

Easy to incorporate - liquid additives. Reduces cure temperature in amino cross-

linked resins systems. Pigment dispersants in anticorrosion coating

formulations for severe applications.

Synthro-cor Am 35 B is easily mixable in water. The metallic salts require a pre-mix procedure before be added into water based systems.

Typical procedure is:

Premix the Synthro-cor with a co-solvent or coalescing agent in the ratio of 50% Synthro-cor by weight plus 45% of co-solvent by weight. Than add 5% of amine by weight or the amine amount necessary to reach a PH of about 8.5.

APPLICATIONS

Coatings

o Automotive OEM and refinish o Industrial maintenance o Coil coatings o Heavy duty coatings o Marine paints

Graphic Arts

o Fountain solutions o Plate cleaners

This pre-mix should be added under good shear forces.

SYNTHRO-COR®

REFERENCESULFONATE SOLVENT %

ACTIVE FEATURES

AM 35 B Amine 2- Butoxyethanoln-Butyl Alcohol

35Environmentally friendly, excel-lent compatibility in water-based and solvent based systems. High efficiency

Ba 50 MS Barium Mineral spirits 50Best compatibility in solvent based system. Highest metal content.

Zn 50 B Zinc 2- Butoxyethanoln-Butyl Alcohol

50Excellent adhesion particularly on zinc treated surfaces.Catalyst for amino resins.

Mg 50 B Magnesium 2- Butoxyethanoln-Butyl Alcohol

50 Hardest film in thermoset coatingsLowest metal content

Ca 50 B Calcium 2- Butoxyethanoln-Butyl Alcohol

50More effective against acidic atmospheres.Excellent pigment dispersant

CA 35 W Calcium 2-Butoxyethanol Ali.Aminoalcohol.

35 Excellent dispersibility in water based systems. Post-addition.

2K acrylic on metal substrate without Synthro-cor AM 35 B

8% anticorrosion pigment in water- based acrylic enamel

8% anti-corrosion pigment in alkyd enamel

2K acrylic on metal substrate

4% anticorrosion pigment 2% Synthro-cor AM 35 B in water-based acrylic enamel

4% anti-corrosion pigment 2% Synthro-cor AM 35 B in alkyd enamel

SSynthro--ccor AM 35 B Liquid corrosion inhibitor to

reduce the need of anti-corrosion pigments in W/B

systems

SSynthro--ccor AM 35 B Liquid corrosion inhibitor to reduce the need of anti-

corrosion pigments in solvent based systems

SSynthro--ccor AM 35 B Liquid corrosion inhibitor to

reduce the need of anti-corrosion pigments in W/B

systems

Synthro-cor® Environmentally friendly anti flash-rust additives.

The chemicals industry is facing strict pressure to decrease human and environmental exposure to chemicals. In response to this pressure, the coating industry is being directed toward more environmentally friendly solutions such as water-borne coatings, even for anti-corrosion applications. This tendency represents a true challenge for the paint formulator. Not only for the need to match the same resistance obtained in the past with solvent based products, but also to deal with problems linked to a water environment, such as flash-rust problems. Flash-rust is an oxidation of the steel substrate that occurs, within minutes, as the water is drying.

Steel naturally oxides when water is present, flash-rust quickly changes appearance to rust spots over a large surface area. The color of the flash-rust may vary depending on the age and composition of the steel and the length of drying.

To solve this problem anti flash-rust additives are added to the paint. These are additives which stop the form of corrosion that occurs during the drying process.

In can protection is also a difficult matter. Even if the inside of the cans are usually treated with a layer of metal resistant to corrosion (such as tin), they maintain a certain porosity that represents a possible point of corrosion attach, especially near the soldering points or rims.

This makes it necessary to use additives able to protect the steel exposed from corrosion either in direct contact with water or either in the empty space of the can where contact with water happens through condensation (vapour phase).

SSynthro--ccor CE 660 B APEO free, VOC free,

Nitrite and nitrate free, Ecolabel compliant,

superior anti flash-rust performances during

application

Without Synthro-cor With Synthro-cor

Synthro--cor B APEO free, VOC free,

nitrite and nitrate free, Ecolabel compliant, anti

flash- rust for in-can protection

Without Synthro-cor B With Synthro-cor B

For samples or more information:

Synthron - 6, rue Barbès – B.P.177 – 92305 LEVALLOIS-PARIS cedex (France) Tel.: + 33 (0) 141341400 – Fax + 33 (0) 141341416

E-mail: postmaster@protex-international.com