What are Corrosion and Scale Inhibitors?

Corrosion and scale inhibitors are tailored chemicals that delay or prevent corrosion and/or scale formation when added in small concentrations in water that would normally create scale deposits. A day-to-day example is prevention of limescale in washing machines. Corrosion is the deterioration and loss of a material and its critical properties due to chemical, electrochemical and other reactions of the exposed material surface with the surrounding environment. In 1936, Professor Langelier’s research best described water corrosion or scale deposition tendency. He gave conditions in which water is balanced with calcium carbonate, making it possible to predict the likelihood of a given water to either precipitate or dissolve calcium carbonate. Scale inhibitors or antiscalants are generally organic compounds containing sulphonate, phosphate, or carboxylic acid functional groups and chelating agents such as carbon, alum and zeolites that sequester and neutralize a particular ion that may be formed. 

So how can scale inhibition be achieved? – Either by adding substances that react with the potential scale-forming substances so that thermodynamically, a stable region is reached or by adding substances that suppress crystal growth. Low dosages of chemicals are used to prevent scale for extended periods for either surface or equipment treatments. Scale inhibitors are most often used as prevention techniques to reduce the scaling risks in near-wellbore locations and wellbore. In this article we seek to answer the following questions:

  • What are the different types of corrosion and scale inhibitors?
  • What are some of the theories behind the mechanism of action?
  • Is there a selection criteria of corrosion and scale inhibitors?
  • What are the advantages of corrosion and scale inhibitors?
  • Are there limitations of corrosion and scale inhibitors?
  • Where are some of the applications of corrosion and scale inhibitors?

What are the different types of corrosion inhibitors?

A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy, that meets the fluid. The chemicals react with the metal surface or the environmental gases causing erosion, thereby, interrupting the chemical reaction that causes corrosion. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime.  Inhibitors can work by adsorbing themselves on the metal’s surface and forming a protective film. 

The different types of corrosion inhibitors include:

  • Anodic inhibitors – metal loss occurs at the anode, so it is important to protect it.
  • Cathodic inhibitors – by acting as sacrificial anodes, they protect the cathode by reacting themselves first, in place of iron or steel.
  • Mixed inhibitors – exhibit both anodic and cathodic protection from corrosion.
  • Volatile Corrosion Inhibitors (VCI) – mainly consist of amino salts or nitrile compounds, it forms a thin barrier on packaging surface to protect metal surfaces by preventing rust and corrosion. 

What are the different types of scale inhibitors?

Scale inhibitors can coarsely be classified as organic and inorganic. The inorganic types include condensed phosphate, such as poly(metaphosphate)s or phosphate salts. Suitable organic scale inhibitors available are poly(acrylic) acid (PAA), phosphinocarboxylic acid, sulfonated polymers, and phosphonates. Phosphonates are maximally effective at high temperatures whereas sulfonated polymers are maximally effective at low temperatures. Copolymers that contain both phosphonate and sulfonate moieties can produce an enhanced scale inhibition over a range of temperatures. 

Scale inhibitors can be classified into three main groups:

  • Thermodynamic inhibitors – complexing and chelating agents, suitable for specific scales. 
  • Kinetic inhibitors – for hydrate formation may also be effective in preventing scale deposition. 
  • Adherence inhibitors – surface active chemicals simply suppress the adherence of crystals to the metal surfaces. 

Two ways by which the kinetic scale inhibitor operates are through adsorption effects and morphological changes of the growing sites. Due to adsorption effects, the inhibitor molecules occupy the nucleation sites which are preferred by the scale forming molecules. Thus, crystals cannot find active places to adhere to the surface and, therefore, crystal nucleation is not promoted. Conventional scale inhibitors are hydrophilic, which means they dissolve in water. It is desirable that the scale inhibitor is adsorbed on the rock to avoid washing out the chemical before it can act as desired. However, adsorption on the rock may change the surface tension and the wettability of the system. To overcome these disadvantages, oil soluble scale inhibitors have been developed. 

What are some of the theories behind the mechanism of action?

The precise mechanism for scale inhibitors is not completely understood but the following are some of the theories. Scale inhibitors may adsorb onto the surface of the scale crystals just as they start to form. The inhibitors are large molecules that can envelop these microcrystals and hinder further growth. This is the primary mechanism. Many oil field chemicals are designed to operate at oil/water, liquid/gas, or solid/liquid interfaces. Since scale inhibitors must act at the interface between solid scale and water, it is not surprising that their performance can be upset by the presence of other surface-active chemicals that compete for the same interface. Before deployment it is important to examine in laboratory tests the performance of a scale inhibitor in the presence of other oil field chemicals. These chemicals function by delaying the growth of scale crystals, the inhibitor must be present before the onset of precipitation. Suspended solids also known as nonadherent scales are not acceptable. This suggests two basic rules in applying scale inhibitors:

  1. The inhibitor must be added upstream of the problem area.
  2. The inhibitor must be present in the scaling water on a continuous basis to stop the growth of each scale crystal as it precipitates.

Is there selection criteria of corrosion and scale inhibitors?

  • Compatibility – The scale inhibitor must not interfere nor be affected by other chemicals such as oxygen scavengers, corrosion inhibitors and biocides. 
  • Application technique – this is the most important if the inhibitor is to be squeezed into the formation. 
  • Severity of scaling – fewer products are effective at high scaling rates. 
  • Efficiency – effective scale control at low inhibitor concentrations.
  • Balanced adsorption-desorption properties – allowing the chemicals to be slowly and homogeneously released into the production water.
  • High thermal stability – higher temperatures and required longer life limit the types of chemistry that are suitable.
  • Environmental considerations – Low toxicity and high biodegradability.
  • pH – most conventional scale inhibitors perform less effectively in a low-pH environment.
  • Viscosity – this is important when considering long umbilical applications such as in remote subsea fields.
  • Cost– sometimes the cheaper products prove to be the most cost effect, sometimes the more expensive products do. 

What are the advantages of corrosion and scale inhibitors?

  • Provide corrosion inhibition in many types of closed recirculation systems 
  • Prevent electrolytic corrosion
  • Protect against cavitation and erosion
  • Protect metal surfaces
  • Cost-effective, easy application and use
  • Offer improved performance 
  • Help to reduce cleaning and maintenance costs 
  • Improve reliability  
  • Optimize operational efficiency 

Are there limitations of corrosion and scale inhibitors?

  • Inorganic corrosion and scale inhibitors suffer hydrolysis and can precipitate as calcium phosphates because of temperature, pH, solution quality, concentration, phosphate type and presence of some enzymes. 
  • Organic corrosion and scale inhibitors suffer hydrolysis with temperature, not effective at high calcium concentrations, must be applied in high doses.
  • Polymer-based corrosion and scale inhibitors have a limited calcium tolerance (2000ppm) although some can work at concentrations higher than 5000 ppm, larger concentrations are needed.
  • EDTA is expensive. 

Where are some of the applications of corrosion and scale inhibitors?

To answer this, we first look at what how do inhibitors function to protect metal surfaces from corrosion? – they function in two ways:

  • They react with the substance or chemical that is the cause of interaction with the metal surface such as removing dissolved oxygen with a chemical reducing agent in solution or in a moist atmosphere.
  • They react with the metal to form a protective layer on the metal surface thereby preventing interaction between the corrosive chemical and the metal. 

Some of the applications of corrosion and scale inhibitors include:

  • Closed-circuit heating and cooling systems.
  • Cooling tower water treatment. 
  • Open recirculating cooling systems.
  • Boiler heat transfer surfaces. 
  • Once through and potable water systems. 
  • Carbon steel equipment in the oil and gas industry.

Corrosion and scale inhibition can take several forms depending on the circumstances of the metal being corroded. Proper monitoring and the elimination of vulnerable surface conditions, to avoid reactive metal combinations are all also part of effective corrosion reduction program. Corrosion and scaling occur as any other chemical reaction, i.e. under the right circumstances, but it can be slowed down using the right strategy of corrosion and scale inhibitors.

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