Immersion Corrosion and Electrochemical Processes Flashcards (46 cards)

What causes corrosion in metals?

Chemical or electrochemical interaction with the environment.

How does erosion differ from friction corrosion?

Erosion is physical destruction; friction corrosion combines physical and chemical effects.

What is rusting in terms of metal corrosion?

Corrosion of iron or its alloys producing hydrated iron oxides.

Can non-ferrous metals rust?

No, they can corrode but cannot rust.

When does electrochemical corrosion occur on metals?

When a metal contacts an aqueous electrolyte, soil, or moist gas.

What causes electrochemical corrosion at different sites?

Local short-circuited corrosion cells with oxidation and reduction.

What defines chemical corrosion in metals?

Reaction with dry gases or non-conducting liquids without electric current.

What components make up an electrode or half-cell?

A metal electronic conductor and an aqueous ionic conductor.

What defines a galvanic cell?

Two electrodes with ion charge transfer and oxidation-reduction at surfaces.

Where does oxidation occur in a galvanic cell?

At the anode electrode.

Where does reduction occur in a galvanic cell?

At the cathode electrode.

What is the oxidation reaction at the anode?

$M \rightarrow M^{z+}+ze^-$

What is the reduction reaction at the cathode?

$M^{z+}+ze^- \rightarrow M$

What is the electrochemical series?

An arrangement of metals by increasing electrochemical potentials.

What is the potential of the normal hydrogen electrode at all temperatures?

Zero volts.

Under what conditions is the normal hydrogen electrode prepared?

Hydrogen at 1013 hPa and 1 mol/dm³ hydrogen-ion activity on platinum black.

What is the Nernst equation for electrode potential?

$E=E^0+\frac{RT}{zF}\ln\frac{a_{\mathrm{ox}}}{a_{\mathrm{red}}}$

What are the values of constants R and F in the Nernst equation?

$R=8.315\ \mathrm{V\cdot C/(K\cdot mol)}$ and $F=96500\ \mathrm{C/equivalent}$.

What happens when a more active metal contacts a less active metal's salt solution?

The more active metal displaces the less active metal from its salt solution.

What is the chemical equation for zinc displacing copper from copper sulfate?

$Zn+CuSO_4\rightarrow ZnSO_4+Cu$

Which metals can displace hydrogen from acids due to their electrochemical potential?

Metals with negative electrochemical potentials.

What is the reaction equation for magnesium with hydrochloric acid?

$Mg+2HCl\rightarrow MgCl_2+H_2$

What happens at more active sites in a local corrosion cell?

They dissolve as anodes.

What is the iron dissolution reaction at the anode in corrosion?

$Fe\rightarrow Fe^{2+}+2e^-$

What is the cathodic reaction in neutral oxygenated environments?

$\frac{1}{2}O_2+H_2O+2e^-\rightarrow 2OH^-$

What does the gravimetric method measure to evaluate corrosion quantitatively?

Mass loss or mass gain of a specimen.

What is the formula for mass-loss corrosion rate $V_C$?

$V_C=\frac{\Delta G}{A\cdot t}$

What units is the mass-loss corrosion rate $V_C$ expressed in?

Grams per square meter per day (g/m²·day).

For what type of corrosion is gravimetry unreliable?

Localized corrosion such as pitting, intergranular corrosion, or dezincification.

Where does corrosion begin on a metal?

At the surface contacting a liquid or gas phase.

What effect does chemical oxidation have on metal surfaces?

It forms a metal-oxide layer that partially protects against corrosion.

How can sulfur-contaminated petroleum affect iron?

It chemically corrodes iron despite low electrical conductivity.

What is a reference electrode?

An electrode with a defined potential to measure test electrode potential.

What is one use of the electrochemical series?

To determine electrochemical cell operation and predict metal reactions with salts and acids.

What industrial process uses displacement of metals by more active metals?

Cementation of metals.

How does copper react with concentrated nitric acid regarding hydrogen release?

Copper reacts without releasing hydrogen.

What products form when copper reacts with concentrated nitric acid?

Copper nitrate, nitrogen dioxide, and water.

Which electrode acts as cathode with different oxygen concentrations?

The electrode in the oxygen-rich solution.

Which regions are anodic in cells caused by temperature differences?

Regions at higher temperature.

What magnifications does an optical microscope use for visual corrosion examination?

10–500× magnification.

What is the magnification range of a scanning electron microscope in corrosion examination?

100–20,000× magnification.

How is linear corrosion rate $V_p$ related to mass-loss rate $V_C$?

$V_p=\alpha V_C$

What is the formula for linear corrosion rate $V_p$ in mm/year?

$V_p=\frac{1000\alpha}{365d}$

What does the laboratory apparatus include besides steel specimens?

100 or 250 ml glass beakers.

What type of solutions are used in the laboratory apparatus?

Acid, base, or salt solutions at specified concentrations.

What sizes are the glass beakers used in the laboratory apparatus?

100 ml or 250 ml.