Corrosion Inhibitor

Introduction

Corrosion inhibitor is chemical substance or mixture of several chemicals that, when added in appropriate concentrations and form to environment (medium), can prevent or slow the corrosion of metals, according to the definition of American Society for Testing and Materials International (ASTM). Metal corrosion has caused huge losses to the global economy. According to a 2016 study by the National Association of Corrosion Engineers (NACE), corrosion costs the world an estimated $2.5 trillion a year, or 3.4% of global GDP. Common methods to prevent metal corrosion include selecting suitable corrosion resistant materials, adopting electrochemical cathodic and anodic protection, coating the metal surface with anticorrosive material, adding corrosion inhibitor, etc. Among them, adding corrosion inhibitor is a simple, convenient, economic and effective method. At present, corrosion inhibitors have widespread application in preventing metal from corrosion in various fields, especially in the fields of the production and processing of petroleum products, chemical cleaning, atmospheric environment, industrial water, machinery, instrument manufacturing and petrochemical production process.

Fig. 1. Corrosion of metal pipe.

Classification

There are many classification ways of corrosion inhibitors.

• According to the chemical composition, corrosion inhibitors are generally divided into three categories of inorganic corrosion inhibitors, organic corrosion inhibitors and polymer corrosion inhibitors. Inorganic corrosion inhibitors mainly include chromate, nitrite, silicate, molybdate, tungstate, polyphosphate, zinc salt, etc. Among them, chromate can be used as corrosion inhibitors for most metals, but it is gradually replaced because of its greater toxicity. Organic corrosion inhibitors mainly include phosphonic acid (salt), phosphonic carboxylic acid, mercaptobenzothiazole, benzotriazole, sulfonated lignin and other heterocyclic compounds. Organic corrosion inhibitors are the most common commercially used inhibitor at present. Polymer corrosion inhibitors mainly include polyethylene, copolymer of phosphonyl and carboxylic acid (POCA), polyaspartic acid and other oligomers chemicals. Usually, the structure of organic corrosion inhibitor and polymer corrosion inhibitor has polar functional groups with oxygen, nitrogen, sulfur and other heteroatoms. These functional groups can react with metal surface to form a protection layer, so that the metal can be isolated from the corrosive medium to get the gain of corrosion inhibition.

Fig. 2. The structure of some representative organic corrosion inhibitors ^[1].

• According to the control position of corrosion inhibitor on electrochemical corrosion, corrosion inhibitor can be divided into anodic corrosion inhibitor, cathodic corrosion inhibitor and mixed corrosion inhibitor. Anodic corrosion inhibitor is achieved the purpose of corrosion inhibition the by inhibiting the anodic process of corrosion reaction. They interact with metal ions to form a protective film in the anodic region of metal, thus inhibiting the dissolution of metal into water. Cathodic corrosion inhibitor mainly inhibits the cathodic reaction of corrosion reaction. They can form a film in the cathodic region of metal to prevent the cathode from releasing electrons. Obviously, the mixed corrosion inhibitor can inhibit both anode and cathode corrosion reactions.

Fig. 3. Scheme of electrochemical corrosion.

• According to the type of metal protected by corrosion inhibitor, corrosion inhibitor can be divided into carbon steel corrosion inhibitor, copper corrosion inhibitor, aluminum corrosion inhibitor, silver corrosion inhibitor and so on. Because of the wide use of carbon steel, the carbon steel corrosion inhibitor is the most used corrosion inhibitor. According to the type of protective film formed by corrosion inhibitor, corrosion inhibitor can be divided into oxidative film corrosion inhibitor, deposition film corrosion inhibitor and adsorbed film corrosion inhibitor. Most of the organic and polymer corrosion inhibitors are adsorbed film corrosion inhibitors, while most of the inorganic corrosion inhibitors are oxidative film inhibitors and deposition film corrosion inhibitors. According to the corrosion medium, corrosion inhibitor can be divided into neutral corrosion inhibitor and acid corrosion inhibitor. Neutral corrosion inhibitors are mostly used in industrial circulating cooling water, and acid corrosion inhibitors are mainly used in fields requiring acid washing liquid such as oilfield exploitation.

Alfa Chemistry can offer various corrosion inhibitors. If you can't find the desired product in our product catalog, we also offer you with custom synthesis service. Please contact us, if you have any needs.

Reference

• Brycki B. E., et al. Organic corrosion inhibitors[J]. Corrosion inhibitors, principles and recent applications, 2018, 3: 33.