Rust preventatives and corrosion inhibitors are lubricants, greases, oils, or fluid additives that form a protective film or barrier to prevent the formation of rust or corrosion. They often have penetrating lubricant or water displacement characteristics. Most antirust agents and moisture-proofing products leaveRust Preventatives and Corrosion Inhibitors Selection Guide a film that prevents water or corrosive chemicals from reaching a metal surface.

 

All metals, with the exception of gold and platinum, corrode in an oxidizing environment. Depending on the specific environment and base metal present, untreated metals will corrode to form compounds such as oxides, hydroxides, and sulfides.

 

Corrosion Inhibitors

 

Corrosion is a natural process where the surface of a substance, typically a metal, deteriorates due to a reaction that occurs within its environment. A corrosion inhibitor is added in small concentrations to the environment in order to control the rate or eliminate corrosion. The efficiency of the inhibitor is dependent on its compatibility with its environment and being able to produce the desired effect when present in small concentrations.

 

Inhibitor efficiency, P, is given as:

 

Inhibitor Efficiency

Where:

 

Corrosion Rate = Corrosion rate

 

Corrosion Rate with Inhibitor     = Corrosion rate with inhibitor in same environment

 


Rust Preventatives

 

Rust preventatives are a specific type of corrosion inhibitor that is applied in order to protect against oxidation. Oxidation is an electrochemical process that occurs on the surface of a metal object. The metal is oxidized, loses electrons, changing its valence state, and reacts with an oxidant, such as oxygen, to form oxides.

 

When a ferrous metal has been oxidized we observe loss of base material and surface deposits of iron oxides. This visible deterioration of the product is commonly referred to as surface rust. Most oxidation-retarding agents use chemicals with ionized particles to slow the oxidation process and prevent the deterioration of products and materials.

 

Classification

 

Corrosion inhibitors are classified by the method by which they provide protection. They either are used to treat the fluid medium the metal is in contact with, vaporize and collect on the treated surface, adsorb to the surface, induce passivation, or provide cathodic protection.

 

Scavengers (Environmental Conditioners)

 

In process applications, treatments may be given to the fluid that a surface is to be exposed to. In these applications a scavenger, often referred to as an environmental conditioner, is used. The scavenger is used to remove or isolate the corrosive species in the fluid medium. In an alkaline or near-neutral solution the scavenger may be used to reduce the oxygen content.

 

Interface Inhibitors

 

Corrosion inhibitors that react with or adhere to the surface of a metallic object are used in most other applications where it is impossible or impractical to alter the environment. Rust Preventatives and Corrosion Inhibitors Selection Guide

 

  • Vapor corrosion inhibitors (VCIs) are typically impregnated into a paper or cloth wrap which surrounds a metal part. The vapor corrosion inhibitor evaporates and forms a protective film that covers the surface of the part. VCIs are typically used in closed environments to offer corrosion protection during storage, transport, or process applications.
  • Adsorption inhibitors, also referred to as mixed inhibitors, adsorb to the surface of the treated material. This is due to the electrostatic attraction between the electrical charge of the metal surface and the ionic charge or dipole of the inhibitor molecules. Adsorption allows the collection of a thin film that does not react with the metal nor corrosive species, but rather acts as barrier that prevents corrosion of the base material.
  • Passivation inhibitors are used to treat a metallic surface. The passivation layer is a thin layer of oxides that forms after a controlled amount of corrosion takes place. The improved surface properties greatly
  • Cathodic precipitators have a higher reduction potential than the metal surface. When present they precipitate insoluble compounds on the metal surface after reacting with the corrosive species.

Industrial Applications

 

In many applications the corrosion inhibitor is delivered in small concentrations through a product that serves additional functions.

 

  • In coolants and heat transfer fluids, the media is diluted in order to prevent the Rust Preventatives and Corrosion Inhibitors Selection Guideformation of corrosive compounds. In this application the corrosion inhibitor is a scavenger that removes corrosive species from the thermal fluid.
  • Paints and coatings may incorporate a corrosion inhibitor. The coating will typically provide improved wear, reduced friction, and improve surface conditions as well as protect the base material from corrosion.
  • Anti-seize compounds are used to reduce friction and to prevent corrosion, galling, and galvanic pitting. Most anti-sticking compounds consist of a solid lubricant dispersed in a grease or binder. The anti-seizing material provides an insulating barrier or layer between metal components.
  • Penetrating lubricants are also useful in the prevention of rust and corrosion. Most penetrating agents are thin, non-evaporative, non-conductive chemicals that can be used to repel moisture or free moving parts that have seized. Some penetrating lubricants are used to protect sensitive electronic components such as printed circuit boards (PCBs) and microchips.
  • Vapor-phase inhibitors are often used when storing or transporting. This may be for automotive parts, mechanical products, or electronic components as to preserve the condition or as a means of protective packaging that isolates the object from external environments.
  • Moisture proofing is used in a variety of commercial, construction, industrial, manufacturing, and engineering applications. Substances that are used to repel moisture include tree and plant resins, rubber, wax, polyurethane, fluorine, acrylic, and polytetrafluoroethylene (PTFE).

Standards

 

ASTM G170 - This guide covers some generally-accepted laboratory methodologies that are used for evaluating corrosion inhibitors for oilfield and refinery applications in well-defined flow conditions.

 

MIL-I-22110 - This specification gives the requirements for volatile corrosion inhibitors (VCI) in crystalline powder form. The corrosion inhibiting vapors from the crystals shall provide corrosion protection for most metals under specific conditions.

 

NACE SP0487 - Provides recommended selection factors, surface preparation requirements, and references to applicable standard evaluation and quality control tests for interim coatings (rust preventives and vapor phase corrosion inhibitors) that provide temporary corrosion protection to parts.

 

Resources

 

Corrosion Inhibitors

 

Corrosion Protection of Metals

 

The Passivation Layer

 

Image Credits:

 

R. S. Hughes Company, Inc. | Daubert Cromwell | Acrolab Ltd.