TOFA-derived corrosion inhibitors: amine salts, the amide route and the film on metal

Fatty acid based corrosion inhibitors are made by forming a salt of tall oil fatty acid with an amine, or by converting it to an amide or an imidazoline. The polar head group adsorbs onto the metal surface while the hydrocarbon tail builds a film that keeps water and oxygen away. This chemistry is used in metalworking fluids and fuel systems.

How is a corrosion inhibitor made from a fatty acid?

The definition of a film-forming organic inhibitor is simple: one end of the molecule must be polar enough to adsorb onto the metal surface, the other non-polar enough to keep water and oxygen away. A long-chain fatty acid meets that definition naturally. The free acid form, however, is rarely used directly, because its dispersion in the water phase, its pH behaviour and its storage stability are not sufficient.

So the fatty acid is used as a raw material in inhibitor production and converted into a derivative. Three routes dominate in practice: an amine salt from neutralisation with an amine, a fatty acid amide from amidation, and an imidazoline obtained by ring closure of that amide. The chain itself stays the same in all three; what changes is the head group that talks to the metal surface.

In the KORKİMYA product data the TOFA grades are listed with metalworking fluid and cutting oil production and with fuel additive production. Corrosion protection is a function delivered inside those two applications; this article covers the chemistry of that function.

The amine salt route: fatty acid and amine together

Neutralisation is the most common route. The carboxyl group of the fatty acid forms a salt with an amine, and the resulting structure is a water-dispersible, surface-active component with alkaline character. In a formulation it does two jobs in one move: it builds the adsorption film on the metal surface and it contributes to the alkalinity of the medium, delaying acidic corrosion.

Amine selection is the second half of that route. Triethanolamine is an alkanolamine listed in the KORKİMYA product data with metalworking fluid and corrosion inhibitor applications, and with corrosion inhibitor among its production areas; its CAS number is 102-71-6. The fatty acid and alkanolamine pair is the classic backbone of inhibitor chemistry in water-miscible systems.

For the formulator the critical point is the stoichiometry of the neutralisation, and that calculation is made directly from the acid value: min 180 for TOFA 180 and min 185 for TOFA 185 (ASTM D1980). The non-reacting fraction is controlled by the unsaponifiables limit: max 6% in TOFA 180 and max 4% in TOFA 185 (ASTM D1065). Those two values determine how much effective inhibitor component is obtained per unit weight.

The amide and imidazoline route: a film that resists water

An amine salt is an ionic structure, which makes it disperse easily in water but limits its resistance to wash-off. Where a more persistent film is needed, the fatty acid is reacted with an amine to form an amide bond. The amide bond is covalent rather than ionic, so it is more resistant to washing with water and to pH swings.

An imidazoline takes that route one step further: the amide formed with a diamine is ring-closed into a five-membered nitrogen-containing ring. That ring makes a strong head group for adsorption onto a metal surface while the long hydrocarbon tail lines up outwards. That is why it is preferred in fuel and hydrocarbon-phase systems.

On the raw material side the governing variables do not change across the routes: free acid content sets the reaction stoichiometry, unsaponifiables set the inactive fraction, and rosin acid content influences colour and adsorption behaviour. Rosin acid is capped at max 3% in TOFA 180 and max 2% in TOFA 185 (ASTM D1240).

How does the film adsorb onto the metal surface?

Corrosion is an electrochemical process running on the metal surface: metal dissolves in one region, oxygen is reduced in another, and ionic conduction links the two. The job of an organic inhibitor is not to stop that circuit chemically but to make it physically harder to run.

The polar head group orients towards and adsorbs onto the active sites on the metal surface. As molecules line up side by side, the hydrocarbon tails order outwards and form a barrier that covers the surface and makes contact with water harder. That barrier slows both the arrival of oxygen at the surface and the departure of dissolved ions.

• Head group: provides adsorption onto the metal surface; it can be an amine salt, an amide or an imidazoline.
• Chain length: sets the packing density of the film and the thickness of the non-polar barrier.
• Unsaturation: affects chain geometry and therefore packing tightness; the iodine value in the TOFA grades is min 150 (ASTM D5768).
• pH and electrolyte load of the medium: the external condition that decides how stable the film stays.

Where it is used: metalworking fluids and fuel systems

In metalworking fluids corrosion protection is not an extra feature but one of the basic requirement lines. The workpiece and the machine itself have to be protected for as long as they are in contact with the fluid. A fatty acid derived component delivers film-forming protection here alongside its emulsion and lubricity functions. The saponification reaction itself and the general structure of a metal cutting fluid formulation are the subject of a separate article on this site and are not repeated here.

On the fuel system side the problem is different: free water in the storage and distribution line causes local corrosion on tank and pipe walls. The structure preferred there is usually a derivative that dissolves in the hydrocarbon phase and forms a film at the interface where water separates. The full fuel additive route, including esterification and lubricity, is covered in a separate article on this site.

A third use is temporary protection: keeping a machined part protected during storage or shipment. Those formulations use the same fatty acid derivatives together with different carrier systems.

Which route for which medium?

The choice among the three routes is made on the water content of the medium, the pH band and how much wash-off the film will face. The table below compares the routes qualitatively; no performance value is claimed.

Which values govern raw material selection?

For an inhibitor producer, the values to read at incoming goods control are the four that determine reaction yield and the colour and stability of the finished product. All of them are reported on the per-lot certificate of analysis.

Inhibitor products that are colour sensitive or require a tight specification take TOFA 185. Where colour tolerance is wide in an industrial formulation, TOFA 180 is the sufficient and more economical choice.

Sample, validation and supply

Corrosion protection is not claimed, it is measured. The right sequence is to run your own derivative synthesis with a sample, evaluate the inhibitor you obtain with your own test protocol, and only then move to a supply agreement.

• CAS numbers: 61790-12-3 (TOFA grades), 102-71-6 (triethanolamine).
• Packaging: ISO tank container 20-24 MT net for TOFA 180 and 20-22 MT net for TOFA 185.
• Storage: 20-40°C, closed tank or container, away from direct sunlight.
• Shelf life: 12 months, extendable to 24 months with quality control testing.
• KKDIK registered; Turkish Safety Data Sheet (SDS/GBF) and a CoA for every lot.