Corrosión en metales. Principios electroquímicos, tipos y prevención

Corrosion in Metals: Understanding the Basics

Introduction to Corrosion

• The class focuses on corrosion in metals, highlighting its significant financial impact, which could be mitigated by understanding theoretical and practical fundamentals.

• Corrosion is defined as the gradual degradation of a material due to environmental factors; metallic corrosion involves chemical or electrochemical reactions with substances in contact with exposed surfaces.

Types and Manifestations of Corrosion

• Corrosion can manifest in various forms, some obvious (like steel oxidation) and others less apparent (such as intergranular corrosion in stainless steel), yet both can cause substantial damage.

• Despite a good understanding of corrosion principles, it results in billions of dollars lost annually worldwide, with 35% of these costs deemed preventable.

Electrochemical Corrosion Process

• Electrochemical corrosion transforms part or all of a metal from a metallic state to an ionic state, driven by electric current flow between different metal parts or areas on the same surface.

• A complete electrical circuit is necessary for current flow during corrosion processes, consisting of four components: anode, cathode, electrolyte, and metallic connection.

Components of the Corrosion Cell

• The anode acts like an electrode where oxidation occurs; electrons flow from the anode through the external circuit while metal ions dissolve into solution.

• The electrolyte is a conductive solution containing charged particles (ions), facilitating electron transport; common electrolytes include acids, alkalis, salts, and especially saline water.

Mechanisms at Anodes and Cathodes

• Different areas within a single piece of metal can act as anodes or cathodes due to internal heterogeneities; impurities may create localized anodic regions leading to localized corrosion.

• A simple corrosion cell consists of an anode, cathode, and electrolyte. Measuring potential differences across these components indicates ongoing current flow between them.

Reactions Involved in Corrosion

• Oxidation reactions at the anode lead to metal dissolution; this process degrades the metal structure over time.

• Reduction reactions occur at the cathode where positive ions react with electrons to form neutral metallic elements without material loss at this site.

Case Study: Iron Corrosion in Pure Water

• In pure water environments lacking dissolved oxygen, iron oxidizes at discrete anodic sites while undergoing specific reduction reactions involving hydrogen ions.

Corrosion Mechanisms and Types

Understanding Corrosion Reactions

• Water is not pure; it contains dissolved oxygen, which can participate in cathodic reactions. The reduction of oxygen leads to the formation of hydroxide ions when reacting with iron.

• Hydroxide ions react with iron ions to form iron hydroxide, which is unstable in oxygenated solutions and further reacts with water and oxygen to produce iron oxide, known for its reddish-brown color.

• Corrosion can occur through various mechanisms simultaneously, some dependent on the environment while others require mechanical or biological assistance.

Common Types of Corrosion

• Uniform corrosion affects the entire surface area of a material progressively, leading to gradual thinning. It represents the most common form of metal loss due to corrosion.

• Other types include galvanic corrosion (between dissimilar metals), erosion (corrosion combined with wear), pitting (localized corrosion), exfoliation, intergranular corrosion, stress corrosion cracking, and fatigue corrosion.

Characteristics of Uniform Corrosion

• Uniform corrosion results from multiple anodic and cathodic areas forming and dissolving over time. It is often caused by atmospheric exposure but can be exacerbated by industrial pollutants or saline environments.

• The rate of uniform corrosion is measured as average thickness loss in millimeters per year. Metals are classified based on their susceptibility: excellent (<0.05 mm/year), satisfactory (0.05–0.5 mm/year), acceptable (0.5–1.25 mm/year), and unsatisfactory (>1.25 mm/year).

Galvanic Corrosion Dynamics

• When two different metals contact each other electrically in an electrolyte presence, a potential difference arises leading to galvanic corrosion where the less noble metal corrodes preferentially.

• The rate of galvanic attack depends on voltage differences between metals, their exposed surface areas, and specific corrosive environments.

Pitting Corrosion Insights

• Pitting occurs as localized attacks resulting in small holes on surfaces; it poses greater risks than uniform corrosion due to difficulty in detection and prediction.

• Pits act as stress concentrators that may lead to sudden material failure; they develop when anodic areas are small compared to protected cathodic areas.

• The penetration rate for pitting can be significantly higher—10 to 100 times more—than that caused by uniform corrosion.

Corrosion Types and Mechanisms

Uniform Corrosion vs. Intergranular Corrosion

• Definition of Intergranular Corrosion: This type of corrosion involves selective attack at the grain boundaries, with minimal impact on the grains themselves. The focus is primarily on the edges or surroundings of the grains.

• Sensitization Process: When grain boundaries become anodic compared to the interior, the metal is said to be sensitized, making it vulnerable to intergranular attack in corrosive environments.

• Example with Stainless Steel: A classic case occurs when stainless steels are sensitized due to chromium diffusion and carbon traces at high temperatures, leading to chromium carbide precipitation at grain boundaries.

Stress Corrosion Cracking

• Mechanism Overview: Stress corrosion cracking (SCC) arises from simultaneous tensile stress, a specific environment, and a susceptible material. It involves subcritical crack growth that leads to eventual fracture.

• Characteristics of SCC Failures: Often manifests as fine cracks penetrating deeply into metals without visible signs of corrosion during casual inspections.

Preventative Measures Against Corrosion

Material Selection and Environmental Control

• Material Choice Importance: Selecting appropriate materials for specific working environments serves as a primary defense against corrosion-related failures.

• Reducing Applied Stresses: Minimizing applied stresses and eliminating residual stresses can significantly mitigate issues related to stress corrosion cracking.

Desalination Processes

• Selective Leaching Explained: This destructive process selectively removes more anodic alloy elements from alloys, resulting in a porous mass. For example, zinc leaching from brass is termed simplification.

• Graphitic Corrosion in Cast Iron: Occurs due to selective removal of iron matrix while leaving behind graphitic flakes when exposed to electrolytes.

Design Practices for Corrosion Prevention

Good Design Practices

• Design Considerations: Effective design practices were discussed as crucial first lines of defense against various forms of corrosion by delaying anodic reactions and reducing overall corrosion rates.

Metal Conditioning Techniques

• Coating Methods Overview: Conditioning metals through coatings or using more corrosion-resistant alloys provides barriers between metals and corrosive environments.

• Types of Coatings: Various coatings include noble metals or protective oxides that act as barriers against corrosive agents; these can be organic or inorganic materials like resins or paints.

Behavior of Protective Coatings

• Zinc vs. Tin Coatings Comparison: Zinc acts sacrificially protecting steel even if scratched; however, tin does not provide similar protection once compromised due to its anodic behavior relative to steel.

Corrosion Resistance and Inhibitors

Key Materials in Corrosion Resistance

• Nickel, chromium, titanium, and zirconium are among the most commonly used corrosion-resistant materials. They react spontaneously with oxygen to form protective films.

• In stainless steel, chromium forms a fine oxide layer (Cr2O3) that protects the metal from corrosion.

Understanding Corrosion Inhibitors

• A corrosion inhibitor is a chemical additive that reduces the rate of corrosion when added to corrosive environments in small concentrations.

• Corrosion inhibitors can be liquid or vapor and include anodic, cathodic, absorption-type, or mixed types. Anodic inhibitors slow down anodic reactions while cathodic inhibitors reduce oxygen levels to hinder cathodic reactions.

Mechanisms of Protection

• Absorption-type inhibitors create a protective film that either physically blocks the corrosive environment or slows down electrochemical processes.

• Electrochemical control is often applied to buried pipes and structures submerged in water for protection against corrosion.

Cathodic Protection Techniques

• Cathodic protection can be achieved using a direct current power source or by applying sacrificial anodes made from metals like aluminum, zinc, or magnesium.