Apr-2005

Corrosion and corrosion enhancers in amine systems

Corrosion in alkanolamine gas treating solutions begins with the acid gases, which are the target of the treating, and is enhanced by several physical and chemical factors.

Arthur L Cummings, Scott W Waite and Dennis K Nelsen
MPR Services Inc

Article Summary

Corrosion causes and enhancers are reviewed, as are suggestions for mitigating or minimising corrosive effects. Particular attention is focused on the effects of amino acids (bicine) because of recent revelations of their presence in a broad range of alkanolamine systems.

Gas processing includes removing acids. Alkanolamine solutions are commonly used to remove acid gas contaminants from various process gas streams in the oil and gas industry, the steel industry, syngas plants, chemical plants and many others. Corrosion is an inevitable result of dealing with acid gases. Much work has been done to understand corrosion mechanisms in alkanolamine (“amine”) systems, so that corrective action may be taken to improve unit operation and reliability.1-14 It is recognised that the major corrosive agents are the very acid gases (H2S and CO2) that are the reason for the existence of amine systems. Other chemicals enter with the gas or are produced within the amine system and contribute to corrosion either directly or indirectly. Physical conditions such at temperature, fluid velocities, suspended solids and metallurgy all play a role.

Theory of corrosion in amine systems
A long list of parameters affecting corrosion in amine systems may be made.13 To reduce the problem to something manageable, parameters are grouped into fewer factors.13 The temptation is to eliminate some factors or groups of factors in order to focus on others. This is very useful, but places limitations on the applicability of the results.

If one is to apply a model to a general 
phenomenon such as amine system corrosion, the model must be robust enough and flexible enough to explain the different varieties of occurrences without straying too far from its foundation. This particular way of viewing corrosion in an amine system follows this requirement.

Simply stated, the corrosion rate in an amine system is a function of four factors. They are corrosive acid concentration, temperature, fluid velocity, and heat stable salt (HSS) anion concentration. Written as a mathematical expression:

corrosion rate = f (acid gas conc., temperature, velocity, HSS anion conc.)

There will also be some dependence on acid gas type and HSS anion type. Amine degradation products (results of chemical modification of alkanolamines) have been characterised as having no practical effect on corronsion8 and as promoting corrosion by complexing iron.5,12,16 Those that complex iron enhance the corrosion in amine systems in the same manner as HSS, so can be included in the HSS term of the expression.

Relating these variables to the physical process of corrosion is needed to validate the expression. In this case, a model view of the physical process of corrosion is required. The model view in this case is built on more fundamental chemical and thermodynamic principles.

The fundamental building blocks are first, the oxidation of iron by some agent, and second, the interaction between the products of corrosion and the corrosive environment. In the case of the typical refinery H2S amine system, the corrosive agent is a form of an acid gas in solution, either a form of H2S in solution or possibly even CO2 in solution. The “form” referred to here is the molecular form of the acid gas. The acid gases mentioned are dibasic; that is, they can give up two protons (hydrogen ions) and form two different ions in solution, as well as exist in the dissolved, hydrated, unionised state. The forms of H2S are hydogen sulphide (H2S), bisulphide (HS-) and sulphide (S=). For CO2, they are carbonic acid (H2CO3), bicarbonate (HCO3-) and carbonate (CO3=). The primary corrosion product referred to in this system is iron sulphide. The great benefit of iron sulfide formation is that this material is fairly insoluble in aqueous solutions and tends to adhere rather uniformly over the surface where the oxidation of the iron took place. If uniformly distributed and with minimal porosity to the solution, the iron sulphide layer affords a substantial barrier to additional corrosion by blocking access of the corrosive agent to the free metal. Iron carbonate is also fairly insoluble, but it does not form as tenacious a layer of corrosion product on the surface of the remaining free metal.

Iron oxidation and protective layer formation
Nature of the acid
The oxidising agents in the amine system are protonic acids (those which give up H+ or a proton). This seems rather odd since the solutions we are talking about are highly buffered, basic solutions. It is also odd because many corrosion control strategies are based on the neutralisation of acids and maintaining a basic pH.

The key to understanding the acid attack then is understanding the acid. First, H2S enters the aqueous phase and is surrounded by water molecules. The H2S then undergoes a two-step reaction sequence, losing an H+ to form H+ and HS- followed by losing the other H+ from the HS- to form H+ and S=. Only an extremely small portion of the original H2S undergoes the second step. Eventually, an equilibrium distribution of the three forms is established.

The final distribution depends on the relative amounts of other acidic or basic species in solution and the amounts of positive or negative charge each contributes. Fortunately, this can be reduced to charge balances and equations using the acid and base dissociation constants. By simultaneously solving the equations, the pH of the solution and the concentrations of any species of any weak acid or weak base constituents can be determined.

In the pH range normally encountered in an amine system, the majority of the H2S in solution will be bisulphide ion, HS-. Under most amine conditions, HS- will be more than 90% of the total H2S in the solution (Figure 1). Now, the question of whether or not HS- can be an acid in a basic solution must be answered.

The answer is yes. It is not that HS- is a strong acid like HCl or H2SO4. Rather, it is that the sulphide also reacts with the oxidised iron to form insoluble iron sulphide. The reaction equation is written: