PTQ Q4 2023 Issue

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

C orrosion 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, corro - sion 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 corro - sion 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 cor - rosion is stopped when oxygen or water/electrolyte is elim - inated 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 sur - face. 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-con - taining 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 conden - sation process, acid components decrease the pH to 1-2. A very high acid concentration (first drop of condensa - tion) 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 sup - ported 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’ 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 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, sav - ing 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 advanta - geous. The opposite effect also occurs if less neutralising amine is needed, and if the dosage rate is reduced, too little filmer may be dosed.

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PTQ Q4 2023