Study guide: Immersion Corrosion and Electrochemical Processes

📋 Course Outline

1. Corrosion Concepts and Classification
2. Electrochemical and Chemical Corrosion
3. Electrodes and Galvanic Cells
4. Electrochemical Series and Potentials
5. Metal Reactions in Solutions
6. Local Corrosion Reactions
7. Non-Electrochemical Corrosion Testing
8. Laboratory Equipment and References

📖 1. Corrosion Concepts and Classification

🔑 Key Concepts & Definitions

• Corrosion: the destruction of metals caused by chemical or electrochemical interaction with the surrounding environment
• Rusting: corrosion of iron or its alloys, producing rust composed mainly of hydrated iron oxides; non-ferrous metals can corrode but cannot rust

★ Must-know

📌 Erosion is the destruction of metals caused by physical phenomena, whereas friction corrosion combines physical effects with chemical interaction.

Further detail

• Corrosion begins at the surface of a metal contacting a liquid or gas phase and progressively advances into the solid phase.

💡 Memory Hook

Corrosion is chemical or electrochemical; erosion is physical.

📖 2. Electrochemical and Chemical Corrosion

🔑 Key Concepts & Definitions

• Electrochemical corrosion: occurs when a metal contacts an aqueous electrolyte, soil, or a moist gas such as atmospheric air
• Chemical corrosion: occurs when dry gases or electrically non-conducting liquids react with a metal without generating or conducting electric current

★ Must-know

• Electrochemical corrosion is produced by local short-circuited corrosion cells in which oxidation and reduction occur at different sites.

Further detail

• Chemical oxidation can form a metal-oxide layer that partially protects the metal against further oxidation and corrosion.

• Sulfur-contaminated petroleum can chemically corrode iron despite having no marked electrical conductivity.

💡 Memory Hook

Electrochemical corrosion needs an electrolyte and current; chemical corrosion does not.

📖 3. Electrodes and Galvanic Cells

🔑 Key Concepts & Definitions

• Electrode: or half-cell, consists of an electronic conductor such as a metal and an ionic conductor such as an aqueous solution containing its ions
• Galvanic cell: a system of two electrodes in which ions transfer charge through the solutions and oxidation and reduction occur at the electrode surfaces
• Reference electrode: has a strictly defined potential and allows the potential of a test electrode to be determined by measuring the electromotive force of a cell

📝 Essential Points

📌 The anode is the electrode where oxidation occurs and electrons are released, whereas the cathode is the electrode where reduction occurs and electrons are accepted.

📐 Formula - At the anode, metal oxidation is represented by $M \rightarrow M^{z+}+ze^-$, while at the cathode, metal-ion reduction is represented by $M^{z+}+ze^- \rightarrow M$.

💡 Memory Hook

The anode oxidizes and releases electrons; the cathode reduces and receives them.

📖 4. Electrochemical Series and Potentials

🔑 Key Concepts & Definitions

• Electrochemical series: the arrangement of metals in increasing order of their electrochemical potentials

★ Must-know

• The normal hydrogen electrode has a potential of zero at all temperatures and is prepared with hydrogen at 1013 hPa and hydrogen-ion activity of 1 mol/dm3 on platinum coated with platinum black.

📐 Formula - The Nernst equation gives the electrode potential as $E=E^0+\frac{RT}{zF}\ln\frac{a_{\mathrm{ox}}}{a_{\mathrm{red}}}$, where $R=8.315\ \mathrm{V\cdot C/(K\cdot mol)}$ and $F=96500\ \mathrm{C/equivalent}$.

Further detail

• The electrochemical series is used to determine the operation of electrochemical cells and predict reactions of metals with salt solutions and acids.

💡 Memory Hook

Reference electrode → potential measurement → metal ranking → reaction prediction.

📖 5. Metal Reactions in Solutions

★ Must-know

📌 A more active metal with a lower electrochemical potential displaces a less active metal with a higher potential from its salt solution.

• Zinc displaces copper from copper sulfate according to $Zn+CuSO_4\rightarrow ZnSO_4+Cu$.

📌 Metals with negative electrochemical potentials can react with acids by displacing hydrogen, as in $Mg+2HCl\rightarrow MgCl_2+H_2$.

Further detail

• Displacement of metals from salt solutions by more active metals is used industrially for cementation of metals.

• Copper reacts with concentrated nitric acid without releasing hydrogen, forming copper nitrate, nitrogen dioxide, and water: $Cu+4HNO_3\rightarrow Cu(NO_3)_2+2NO_2+2H_2O$.

💡 Memory Hook

Lower potential → more active metal → displacement of the less active metal.

📖 6. Local Corrosion Reactions

★ Must-know

• In a local corrosion cell, more active surface sites dissolve as anodes while less active sites act as cathodes, with the sites electrically short-circuited through the metal.

📐 Formula - Iron dissolution at the anode is $Fe\rightarrow Fe^{2+}+2e^-$, hydrogen-ion reduction at the cathode is $2H^++2e^-\rightarrow H_2$, and the overall reaction is $Fe+2H^+\rightarrow Fe^{2+}+H_2$.

📐 Formula - In neutral oxygenated environments, the cathodic reaction is $\frac{1}{2}O_2+H_2O+2e^-\rightarrow 2OH^-$.

Further detail

📌 For identical metal electrodes exposed to solutions with different oxygen concentrations, the electrode in the oxygen-rich solution generally acts as the cathode.

📌 In cells caused by different temperatures, regions at higher temperature are usually anodic.

💡 Memory Hook

Surface heterogeneity → local galvanic cells → anodic dissolution and cathodic reduction.

📖 7. Non-Electrochemical Corrosion Testing

🔑 Key Concepts & Definitions

• Gravimetric method: quantitatively evaluates corrosion by measuring the mass loss or mass gain of a specimen

★ Must-know

📐 Formula - The mass-loss corrosion rate is calculated as $V_C=\frac{\Delta G}{A\cdot t}$, where $V_C$ is in g/m²·day, $\Delta G$ is the mass change, $A$ is the exposed area, and $t$ is the corrosion time.

📌 Gravimetry is reliable for general or surface corrosion but fails to characterize localized corrosion such as pitting, intergranular corrosion, or dezincification.

Further detail

• Visual corrosion examination is qualitative and can use an optical microscope at 10–500× magnification or a scanning electron microscope at 100–20,000× magnification.

📐 Formula - Linear corrosion rate is related to mass-loss rate by $V_p=\alpha V_C$ and expressed as $V_p=\frac{1000\alpha}{365d}$ in mm/year, where $d$ is material density in g/cm³.

💡 Memory Hook

Visual microscopy is qualitative; gravimetry gives quantitative mass-loss data.

📖 8. Laboratory Equipment and References

📝 Essential Points

• The laboratory apparatus consists of: different steel specimens, 100 or 250 ml glass beakers, an acid, base, or salt solution at a specified concentration

📊 Synthesis Tables

Electrochemical versus chemical corrosion

Feature	Electrochemical corrosion	Chemical corrosion
Environment	Aqueous electrolyte, soil, or moist gas	Dry gas or non-conducting liquid
Electrical current	Local corrosion cells with charge transfer	No generation or flow of electric current
Main mechanism	Coupled oxidation and reduction	Direct chemical reaction
Typical product	Corrosion products formed through electrode processes	Oxide or other product layer that may slow further attack

Corrosion assessment methods

Method	Measurement	Main limitation
Visual microscopy	Surface appearance and corrosion sites	Qualitative only
Gravimetry	Mass loss or mass gain per area and time	Poor for localized corrosion
Linear-rate calculation	Dimensional loss in mm/year	Derived from mass loss and density