One of the production-related problems most commonly encountered in any industry is the issue of metal corrosion, which renders expensive equipment inoperable. Metal corrosion is the process of metal ion oxidation, which eventually leads to its destruction. The issue of metal corrosion is especially pressing in the chemical and oil processing industry, where equipment is often in regular contact with aggressive acidic media that speed up the process of corrosion to a considerable extent. The solution to this situation lies in the usage of metal corrosion inhibitors, which inhibit the process essentially.
Corrosion inhibitors are chemically active elements capable of slowing down the corrosion process thanks to certain natural properties. Typically, corrosion inhibitors are added to the medium that causes corrosion, thus retarding the process of metal decay, but a certain type of inhibitors, the contact inhibitors, require immediate contact between the reagent and metal surface.
Metal corrosion is an electrochemical process of metal dissolution in the environment, which inflicts irreparable damage upon various industrial mechanisms. The process of corrosion can be divided into two main stages: the anode process and the cathode process. Metal corrosion inhibitors have a direct effect on the electrochemical process that causes corrosion, which results in a significant decrease in the corrosion rate.
There are several types of media that have corrosive influence on metals: non-aggressive, mildly aggressive and aggressive media. The effect of different metal corrosion inhibitors may vary depending on the medium. Corrosion inhibitors are divided into several types, depending on the most appropriate medium: acidic, neutral and alkaline corrosion inhibitors.
DIFFERENT TYPES OF METAL CORROSION INHIBITORS
Film metal corrosion inhibitors are adsorbed onto the treated metal surface. As a result of adsorption, the corrosion inhibitor forms a thin sustainable film that covers the entire treated surface. The corrosion inhibitor film effectively prevents the contact between corrosive media and metal surface, thus preventing metal decay.
Passivation metal corrosion inhibitors act using a different principle. Through reacting with metal ions, corrosion inhibitor forms stable insoluble compounds — a layer of oxide, which also covers the surface of metal with a protective film. Thus, metal does not come into contact with water and water is indirectly purged of iron. Therefore, oxidating corrosion inhibitors act in a manner similar to that of film inhibitors, preventing the environment from affecting the metal and thus restricting its corrosive influence.
As distinct from the former two types, cathodic metal corrosion inhibitors use a completely different principle to protect metal against corrosion. Corrosion inhibitors of this kind increase the tension of cathodic process, one of the stages of metal corrosion, which results in a considerable decrease in the rate of decay.
The choice of an optimum metal corrosion inhibitor depends directly on the properties of the inhibitor itself, properties of the metal being treated and the characteristic and conditions of the medium that the corrosion inhibitor and the treated metal surface are going to contact with. Now, let us take a closer look at the properties, characteristics and action principles of corrosion inhibitors of different kinds.
ADSORBING METAL CORROSION INHIBITORS
Physical adsorption of metal corrosion inhibitors is determined by the presence of Van der Waal’s forces between the treated metal surface and the corrosion inhibitor. At that, the film that occurs between the metal and corrosion inhibitor is easily washed away.
Chemical adsorption of metal corrosion inhibitors is based on other principles. Inorganic corrosion inhibitors are defined as various volatile substances that evaporate upon contact with the environment. Vapours of volatile inhibitors come into contact with the water film on the protected metal surface and dissolve therein. Thus, the water film, saturated with metal corrosion inhibitor, is adsorbed onto the surface of metal, preventing the influence of corrosive medium thereupon. This method of protecting metal with corrosion inhibitors is also deemed an adsorption method.
Various amines and derivatives thereof, alcohols, carbamides, nitrites and carbonates are deemed to be the basic adsorption-based metal corrosion inhibitors. The recommended method for selecting the most appropriate most corrosion inhibitor is the performance of practical tests that would determine the advisability of using a certain corrosion inhibitor in each specific set of conditions. One must also keep in mind that the efficiency of metal protection is affected not just by the properties of the inhibitor itself, but by the properties of treated metal and environmental factors like pH level, temperature, etc. as well.
PASSIVATION METAL CORROSION INHIBITORS
Passivation inhibitors are somewhat similar to adsorbing corrosion inhibitors in terms of action principle — these form a protective film that hampers the influence of aggressive corrosive medium on metal. As distinct from adsorption-based methods, however, such metal corrosion inhibitors do not form sediments on the surface of metal, but create low-solubility compounds with the ions of metal itself through chemical reactions. This covers the surface of metal with an oxide layer that prevents corrosive influence on metal. Inhibitors can be supplied using chemical dispensing pumps.
The benefit such metal corrosion inhibitors are deemed to provide is that the protective film is hard to remove, which ensures effective protection of metal against corrosive influence of the environment. Formation of low-solubility compounds from corrosion inhibitors and metal ions yields high metal protection efficiency, so metal surface takes virtually no damage.
USAGE OF METAL CORROSION INHIBITORS
Efficiency of metal corrosion inhibitors is, first of all, a function of the ability thereof to alter the kinetics of electrochemical reactions, which include corrosion. Metal corrosion inhibitors can be divided into three types, by action principle: anodic, cathodic and mixed corrosion inhibitors. Metal corrosion consists of two main stages – anodic and cathodic. These are the processes hampered by metal corrosion inhibitors. One of the main aspects when using inhibitors is dose accuracy, so it is advised to use dispensing pumps or a reagent dosage system.
Anodic inhibitors include reagents like chromates and nitrites, i.e., passivation inhibitors. As set out in the definition thereof, passivation corrosion inhibitors react with metal ions and form low-solubility compounds that cover the surface of metal like a protective film. Nitrates and chromates are oxidants, so metal ions oxidize upon contact with these corrosion inhibitors, which results in the formation of an oxide film.
Cathodic metal corrosion inhibitors include such compounds as arsenic and bismuth salts and various organic compounds that conduce hydrogen overvoltage on the surface of metal. The elements holding a specific place among cathodic metal corrosion inhibitors are hydrazine, sodium sulphite and other corrosion inhibitors that consume cathodic depolarizers.
Most metal corrosion inhibitors are applied by means of introduction into water that is going to come in contact with metal surfaces, various mechanisms and devices. Various instruments like dispensing pumps or membrane pumps are used to ensure precise dosage of metal corrosion inhibitors. Aside from measuring the required amounts of corrosion inhibitors, these also mix water properly, thus distributing the corrosion inhibitor evenly within the entire volume of water.
Some corrosion inhibitors also act as salt formation inhibitors, preventing salification that poses a considerable hindrance to the operation of water supply systems and destroys metal surfaces.
Double action metal corrosion inhibitors are created by mixing various reagents that are “responsible” for slowing down a specific chemical reaction.
Choosing an optimum metal corrosion inhibitor.
In order to choose a metal corrosion inhibitor that would meet all the requirements of each specific situation, one must not just carry out a complete chemical assay of water to reveal the presence or absence of substances capable of affecting the procedure of reaction therein, but also ascertain parameters like the properties of the inhibitor and the metal to be protected. In order to reduce the consumption of inhibitors, one can consider water deferrization.
Before the purchase of inhibitors, one should analyse the water in order to determine the acid-alkaline balance thereof. By the mode of usage, inhibitors are known to be divided into three types: acidic corrosion inhibitors, alkaline corrosion inhibitors and neutral corrosion inhibitors. Before the purchase of inhibitors, one should pay attention to the adequacy of the inhibitor type to the acid-alkaline balance of water in each specific set of conditions. Usage of inhibitors has no water softening effect. Inhibitors only protect metal against corrosion.
The mode of action of the corrosion inhibitor itself is of no lesser importance. Before buying an inhibitor, be advised to calculate the compatibility of corrosion inhibitor action features and the properties of metal itself, as this is what the efficiency of corrosion inhibitor-assisted metal protection depends upon.
The primary criterion for the choice of a corrosion inhibitor is the metal itself, as its properties have a direct effect on the efficiency and action of metal corrosion inhibitors.
For example, in order to protect non-ferrous metals from corrosion, one is advised to purchase benzotriazole- or chromate-based inhibitors for copper, or bichromate for the protection of silver. Sodium nitrite is vastly popular as a means to protect ferrous metals against corrosion, along with a number of volatile corrosion inhibitors. In order to protect metals from atmospheric corrosion, one is advised to purchase contact corrosion inhibitors that are used to treat the surface of metal.
Therefore, in order to purchase an inhibitor that could stop or reduce corrosion to a considerable extent, one must first determine the nature and properties of the metal to be protected, then analyse the environment that is going to affect metal and select and buy a corrosion inhibitor that meets these data.