Control corrosion in refineries and petrochemical plants

Formation of a water-resistant organic film barrier stops electrochemical corrosion on metal surfaces.

Berthold Otzisk, Kurita Europe
Mohamed Hudhaifa, Kurita AquaChemie

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Article Summary

Corrosion protection and prevention is an endless fight to prolong the runtime of process units with extended equipment lifetime. Special cladding or coatings of vessels, pipes, and distillation columns can help, reducing the corrosion potential. Alternatively, corrosion inhibitor programmes are mainly to be used with less effort. Oxygen scavengers or film-forming amines (FFAs) have proven their worth in refineries and petrochemical industries for many years. They help avoid the risk of corrosion or at least reduce the risk of further corrosion attacks.

Aqueous corrosion is often described as a corrosion cell with the condition on a metal surface in which a flow of electric current occurs between an electrolyte and the metal surface to cause the metal to degrade. Electrochemical corrosion is stopped when oxygen or water/electrolyte is eliminated to break the corrosion cell. The formation of a thin film layer on the metal causes the function of a corrosion cell to break. The filmers absorb onto the metal surface by forming a water-resistant organic film barrier on the surface. This prevents the acid components from contacting the metal surface and reduces the corrosion risk.

Protective films
Oil-soluble filming corrosion inhibitors are often amine-containing molecules that build up a protective film, which is why the term ‘filming amine’ has become established. However, there are also film-forming corrosion inhibitors that do not contain nitrogen as an amine in the molecule. Most filming corrosion inhibitors are designed as ready-made formulations to be effective across a broad pH range. The corrosion rate increases as the pH decreases, and the coating performance must be effective even in the low pH range. At overhead systems of distillation columns, hot gases are cooled down, and at the beginning of a condensation process, acid components decrease the pH to 1-2.

A very high acid concentration (first drop of condensation) is observed, and the corrosion inhibitor must build up adequate protection until condensation with a collection of sour water happens at a far higher pH in the accumulator drum. The protective filming function is sometimes supported by the addition of a neutralising amine so that the pH value is raised more quickly to a higher level.

Filming corrosion inhibitors are usually oil-soluble additives that are corrosive even as a pure application product, as noted in the material safety data sheet (MSDS). Diluted into a hydrocarbon-containing product stream, the polar part of the active substance then attaches itself to the metal surface to form the barrier with the non-polar part in combination with the liquid hydrocarbons. Imidazoline derivatives, naphthenic acid salts, or so-called ‘fatty acid, tall-oil reaction products with amines’ are commonly used as active substances. There are other very effective active substances that cannot be discussed further here.

When using oil-soluble filming additives, it is important to be cautious and avoid overdosing. Typically, there are no issues when injected in low ppm concentrations. Due to the often high boiling temperature of active ingredients, overdosing may result in a stable emulsion or foam, making excess filmer difficult to remove. The saying ‘less is more’ applies here. Droplet entrainment downstream or upstream into products where no filmer should be dosed can lead to problems or quality issues. These corrosion inhibitors have dispersing properties at high concentrations, which can remove the natural protective layer on the metal surface.

A filmer can be combined with a neutralising amine, saving a dosing point with an extra dosing pump, container, and dosing lines. However, one disadvantage is that both active substances (filmer plus neutralising amine) always need to be increased or reduced simultaneously. If, for example, the pH in the sour water is too low and more neutralising amine has to be dosed, the dosing rate for the filmer is inevitably increased. This is not always advantageous. The opposite effect also occurs if less neutralising amine is needed, and if the dosage rate is reduced, too little filmer may be dosed.
Water-soluble filming corrosion inhibitors

For several years, the need for water-soluble filmers has become increasingly important. The average volume of water recovered from crude oil rises to provide superior protection in systems with high water cuts. High system pressures increase the concentration of dissolved gases. Carbon dioxide forms carbonic acid, which reacts with the metal to form iron carbonate. This leads to pitting corrosion and weakens the equipment. Sulphate-reducing bacteria in some crude oils can lead to the formation of hydrogen sulphide gas. It is a corrosive gas and reacts with steel to form iron sulphide with pitting corrosion.

Like oil-soluble filmers, the water-soluble additives build a protective barrier to keep corrosive components away from the metal surface. In this case, however, no hydrocarbon-containing media is needed, as the non-polar components of the filmer combine and thus build up the barrier. This effect can then be used as a protective function in systems with high water cuts.

Imidazoline salts, naphthenic acid salts, alkoxylated phosphate esters or sarcosines are often used as corrosion inhibitors for aqueous systems. Chemically, imidazolines belong to heterocyclic compounds. They have a five-membered ring structure with two nitrogen atoms in the ring. Many well-established imidazolines are available on the market, and the imidazoline molecules have different structures and hydrocarbon chain lengths. Oil-soluble imidazolines can be converted into the corresponding water-soluble imidazoline salt by reaction with an acid.

However, weaker acids are preferred to be used and not hydrochloric acids to form the corresponding chloride salts. At higher temperatures, chlorides may be knocked out of the molecule and thus become very corrosive again. Therefore, it is better to use corrosion inhibitors with alkylammonium chlorides only at lower temperatures as active ingredients.

At higher dosages, water-soluble filmers tend to form stable emulsions or foam. For this reason, special care must be taken to avoid overdosing when using this type of filmers. It is of great advantage that water-soluble filmers can be combined well with Kurita’s ACF Technology. ACF Technology uses solid organic bases that react immediately with corrosive acids to form an ACF salt with very low corrosion potential and a neutral pH of 7. Already formed ammonium chloride or ammonium sulphide deposits, which have a very high corrosion potential, are dissolved in the running process and removed as water-soluble ACF salts with the sour water. With a combined application of a filmer and ACF technology, the metal surface can be protected against corrosion, and the deposition of ammonium salts can be prevented simultaneously.

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