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An Introduction To Corrosion Inhibitors




An Introduction To Corrosion Inhibitors Dear All,
 
There have been quite a few threads devoted to injection of corrosion inhibiting chemicals on "Cheresources". Today's blog entry makes an attempt to give some introductory knowledge related to "Corrosion Inhibitors".
 
The following paragraphs provide the details. The reference for the content of this blog article is provided at the end of the blog article:
 
The use of chemical inhibitors to decrease the rate of corrosion processes is quite varied. In the oil extraction and processing industries, inhibitors have always been considered to be the first line of defense against corrosion. A great number of scientific studies have been devoted to the subject of corrosion inhibitors. However, most of what is known has grown from trial and error experiments, both in the laboratories and in the field. Rules, equations, and theories to guide inhibitor development or use are very limited. By definition, a corrosion inhibitor is a chemical substance that, when added in small concentration to an environment, effectively decreases the corrosion rate. The efficiency of an inhibitor can be expressed by a measure of this improvement:
 
Inhibitor Efficiency (%) = 100*(CRuninhibited -CRinhibited) / CRuninhibited
where:
CRuninhibited = corrosion rate of the uninhibited system
CRinhibited = corrosion rate of the inhibited system

 
In general, the efficiency of an inhibitor increases with an increase in inhibitor concentration (e.g., a typically good inhibitor would give 95% inhibition at a concentration of 0.008% and 90% at a concentration of 0.004%). A synergism, or cooperation, is often present between different inhibitors and the environment being controlled, and mixtures are the usual choice in commercial formulations. The scientific and technical corrosion literature has descriptions and lists of numerous chemical compounds that exhibit inhibitive properties. Of these, only very few are actually used in practice. This is partly because the desirable properties of an inhibitor usually extend beyond those simply related to metal protection. Considerations of cost, toxicity, availability, and environmental friendliness are of considerable importance.
 
Attached Table presents some inhibitors that have been used with success in typical corrosive environments to protect the metallic elements of industrial systems. Commercial inhibitors are available under various trade names and labels that usually provide little or no information about their chemical composition. It is sometimes very difficult to distinguish between products from different sources because they may contain the same basic anti-corrosion agent. Commercial formulations generally consist of one or more inhibitor compounds with other additives such as surfactants, film enhancers, de-emulsifiers, oxygen scavengers, and so forth. The inhibitor solvent package used can be critical in respect to the solubility/dispersibility characteristics and hence the application and performance of the products.

 
Attached Image
 
Classification of Inhibitors
Inhibitors are chemicals that react with a metallic surface, or the environment this surface is exposed to, giving the surface a certain level of protection. Inhibitors often work by adsorbing themselves on the metallic surface, protecting the metallic surface by forming a film. Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes by:
 
-  Increasing the anodic or cathodic polarization behavior (Tafel slopes)
-  Reducing the movement or diffusion of ions to the metallic surface

-  Increasing the electrical resistance of the metallic surface
 
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality, as follows:
 
Inorganic inhibitors: Usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the negative anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
 
Organic anionic: Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and antifreeze solutions.
 
Organic cationic: In their concentrated forms, these are either liquids or waxlike solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
 
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors in a functionality scheme as follows:

 
Passivating (anodic)
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range. There are two types of passivating inhibitors: oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and the non-oxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel.
 
These inhibitors are the most effective and consequently the most widely used. Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of application (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary, sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring. In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential that monitoring of the inhibitor concentration be performed.
 
Cathodic
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas. Cathodic inhibitors can provide inhibition by three different mechanisms: (1) as cathodic poisons, (2) as cathodic precipitates, and (3) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult. Other cathodic inhibitors, ions such as calcium, zinc, or magnesium, may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulfite (Na2SO3).

 
Organic
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface. Their effectiveness depends on the chemical composition, their molecular structure, and their affinities for the metal surface. Because film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulfonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors.
 
These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrates, the concentration of the inhibitor in the medium is critical. For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water with a pH of 7.5 and containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor.

 
Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of precipitates on the surface of the metal, thereby providing a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.
 
The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of rust water. In aerated hot water systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where nontoxic additives are required.

 
Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapor phase inhibitors (VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapor spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine, and hexamethylene-amine are used. On contact with the metal surface, the vapor of these salts condenses and is hydrolyzed by any moisture to liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of these compounds, fast action wanting high volatility, whereas enduring protection requires low volatility.

 
I would enjoy having a discussion and some insight on "corrosion inhibitors" from knowledgeable members of the forum.
 
Reference: Chapter 10, Corrosion Inhibitors" "Handbook of Corrosion Engineering" by Pierre R. Roberge
 
Regards,
Ankur

 




Photo
theleftcoast
Mar 06 2014 09:56 PM

Thanks for the article Ankur.  Good read.  Great reference to the Handbook of Corrosion Engineering.  I checked out the book on Amazon and will definitely get it.

 

I work as a production engineer (oil and gas) and HCL used in acid jobs commonly has an inhibitor package added.  For HCL in this application, what is the most common inhibitor used (excluding other additives used in oil well acidizing)?

 

John

John,

 

Since I don't work in an industry related to production, storage and transport of HCl, I am not in a position to tell you what corrosion inhibitors they use. As a general knowledge, I do know that a lot of process industries where HCl is stored, handled and transported, plastic tanks and pipes are used and any steel systems are lined with plastic / polymer linings. The link below provides storage and handling practices for HCl:

 

http://www.solvayche...03-W-EN_WW_.pdf

 

Regards,

Ankur.

Photo
alberto.palma1972
Mar 24 2014 01:58 AM

Defenetively plastic pipe is the correct choice for HCl, when applicable, since it tends to be very aggrssive to steel pipe even in low concentration. If temperature constraint doesn't allow plastic pipe, the choice are more exotic material like titanium and alloys.

Alberto

Photo
javad_mohseni
Apr 29 2014 11:45 AM
hi mr admin.my name is farzad .nice to meet you.im an mechanic engineer
i i have a question.infact i have a problem.i dont know how can i to find  precharge nitrogen gas to pulsation dampeners..i dont know how can i find precharge set for pulsation dampeners for reciprocating pumps.
for example: 
specification:
metering pump-a01  datasheet
suction pressure:1.5bar
discharge pressure:2.5bar
capacity:25lit/hr
 pulsation dampener precharge set pressure for this pump:(?)
 
 please give me some alternative for find this set for diffrent reciprocating or metering pumps
 
thank you mr admin

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