Understanding Metal Corrosion: Processes and Prevention

Posted on Nov 25, 2024 in Chemistry

Understanding Metal Corrosion

Metal corrosion is the transformation of a metal or metal alloy through chemical or electrochemical interactions, resulting in corrosion products and energy release.

Metallic corrosion (electrochemical mechanism) typically occurs when metal is exposed to water molecules, oxygen gas, or hydrogen ions in a conductive medium.

Protection Against Metal Corrosion

Protection methods must consider technical and economic aspects. Exposure is crucial. Corrosion inhibitors or controlling aggressive agents (SO₂, H⁺, Cl^–) are impractical for atmospheric corrosion. Cathodic protection is also limited, leaving metal modification or barrier interposition as alternatives.

Historically, corrosion meant oxidation with oxygen. With atomic structure knowledge, it was understood that oxidation involves electron loss, not necessarily with oxygen (e.g., Al⁰ → Al⁺³ + 3e^–).

Corrosion Types and Processes

Corrosion is material deterioration by chemical or electrochemical means, possibly with mechanical stress. Materials used in equipment or facilities must resist corrosive environments and have sufficient mechanical and manufacturing properties. Corrosion affects metals (steel, copper alloys) and non-metals (plastics, ceramics, concrete). This focuses on metallic corrosion.

Corrosion processes are classified into two major groups:

• Electrochemical Corrosion
• Chemical Corrosion

Electrochemical Corrosion

Electrochemical corrosion requires:

• Liquid water
• Temperatures below water’s dew point (mostly room temperature)
• Formation of a corrosion cell with electron movement on the metal surface

Electrochemical corrosion is also called aqueous corrosion due to the need for liquid water. Metals react with non-metals (O₂, S, H₂S, CO₂) to form compounds similar to those extracted, reversing metallurgical processes (see Figure 1).

FIGURE 1

Chemical Corrosion

Chemical etching processes are also known as high-temperature corrosion or oxidation. These processes are less common in nature and involve high temperatures.

Characteristics of chemical corrosion:

• Absence of liquid water
• High temperatures, above water’s dew point
• Direct interaction between metal and corrosive medium

Chemical corrosion is also called non-aqueous or dry corrosion.

Material deterioration not meeting the corrosion definition includes erosion, which mechanically removes particles. While gradual and caused by the environment, it’s physical, not chemical or electrochemical. Corrosion-erosion occurs when both processes happen simultaneously.

Metallurgical transformations during service, especially at high temperatures, can alter mechanical properties (e.g., increased brittleness). These changes are not corrosion but can affect corrosion resistance, leading to intergranular corrosion. Creep, a time-dependent deformation under stress and temperature, can also occur.

Corrosive Media

Electrochemical corrosion requires an electrolyte, a conductive solution of water with salts, acids, or bases.

Main Corrosive Media and Electrolytes

• Atmosphere: Contains moisture, salts, industrial gases, dust. Electrolyte is water condensing on metal with salts or gases.
• Soil: Contains moisture, minerals, bacteria, and may be acidic or basic. Electrolyte is water with dissolved salts.
• Natural Waters (rivers, lakes, underground): Contain minerals, acids, bases, industrial waste, bacteria, pollutants, and gases. Electrolyte is water with dissolved salts.
• Sea Water: Contains significant salts (see table below). Electrolyte is water with high salt content. Dissolved gases can accelerate corrosion.

• Chemicals: Chemicals in contact with water or moisture can form electrolytes and cause electrochemical corrosion.

Corrosion Reactions and Products

Corrosion involves electrochemical oxidation and reduction reactions.

Anodic reactions (anode of corrosion cell) are oxidation reactions:

(Metal wear)

Cathodic reactions (cathode of corrosion cell) are reduction reactions:

Common cathodic reactions are “a”, “b”, and “c”. Reactions “d” and “e” are less frequent, with “e” occurring only in chemical or electrolytic reduction.

Cathodic reactions in neutral, aerated media:

Cathodic reactions in neutral, non-aerated media:

Major Findings

• The cathode region becomes basic (pH increases).
• In aerated media, H₂ is absorbed, causing hydrogen overvoltage and delaying corrosion (cathodic polarization).
• In aerated media, O₂ consumes H₂, preventing hydrogen overvoltage and accelerating corrosion.

Electrolyte composition around the cathode depends on oxygen diffusion and electrolyte renewal rate. Reaction “a” can occur in aerated media with high electron flow, as in cathodic overprotection in seawater, where reaction “c” is overwhelmed. This can cause hydrogen embrittlement, leading to cracks or reduced fatigue life.

Note: Acidity decreases around the cathode in acidic media, and alkalinity increases in basic media.

Corrosion products in electrochemical processes are usually insoluble compounds of metal ions and hydroxyl ions, forming metal hydroxides or hydrated metal oxides. Other ions in the corrosive medium can form sulfides, sulfates, chlorides, etc.

Polarization, Passivation, and Corrosion Rate

Polarization

Polarization is the electrode potential modification due to concentration changes, gas overvoltage, or ohmic resistance variation.

Polarization reduces current between anodes and cathodes, preventing a short circuit due to low metal and electrolyte resistance.

Polarization brings anodic and cathodic potentials closer and increases ohmic resistance, limiting corrosion rate.

Corrosion rates are lower than expected due to polarization.

Anodic control: Corrosion reaction controlled by anodic polarization.

Cathodic control: Reaction controlled by cathodic polarization.

Ohmic control: Reaction controlled by increased contact resistance.

Corrosion reactions usually have mixed control.

Polarization causes:

A – Concentration Polarization

This type of polarization occurs frequently in electrolytes stopped or slow moving.

The polarization effect due to increased concentration of metal ions around the anode area (lowering its potential in the table of potential) and rarefaction of H + ions in the surrounding area cathode.

If the electrolyte movement has both situations should not happen.

B – ACTIVATION BY POLARIZATION

This type of polarization occurs due to overvoltage gas surrounding the electrodes.

The most important cases in the study of corrosion, are those where a release of H2 in the vicinity of the cathode or O2 in the vicinity of the anode.

The release of H2 in the vicinity of the cathode is called cathodic polarization and is of particular importance as a factor controlling the corrosion process.

In aerated electrolytes little H2 released and absorbed into the cathode area causes a surge or overvoltage of hydrogen capable of significantly reducing the aggressiveness of the medium. Can be considered for this fact the corrosion of steel negligible in the presence of salt or fresh water, fully degassed.

The hydrogen overvoltage was investigated by Tafel stating the following equation:

where:

I

– Overvoltage of hydrogen, V; In V and In A / cm 2 – constants that depend on the metal and environment; – Applied current density which causes overvoltage In A / cm 2.

– Figure 01
Tafel curve
On voltage as a function of current density

C – POLARIZATION ohmic

The ohmic polarization is due to precipitation of compounds that become insoluble with increasing pH in the surrounding areas of the cathode.

These compounds are mainly carbonates and hydroxides that form a natural coating on the cathodic areas, mainly calcium carbonate and magnesium hydroxide.