Corrosion 3&4

Question 1

What is galvanic corrosion?

Answer 1

Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte.

Question 2

When does galvanic corrosion occur?

Answer 2

Galvanic corrosion occurs when two dissimilar metals are electrically connected in an electrolyte.

Question 3

What causes galvanic corrosion?

Answer 3

Galvanic corrosion results from a difference in oxidation potentials of metallic ions between two or more metals. The greater the difference in oxidation potential, the greater the galvanic corrosion.

Question 4

What is galvanic coupling?

Answer 4

Galvanic coupling is a galvanic cell in which the anode is the less corrosion-resistant metal compared to the cathode.

Question 5

What happens to the less noble metal in galvanic corrosion?

Answer 5

The less noble metal will corrode and act as the anode, while the more noble metal will not corrode and act as the cathode.

Question 6

What determines the nobility of metals and semi-metals in galvanic corrosion?

Answer 6

The galvanic series (or electro-potential series) determines the nobility of metals and semi-metals.

Question 7

What factors determine the rate of galvanic corrosion?

Answer 7

The rate of galvanic corrosion is determined by the electrolyte and the difference in nobility between the two metals.

Question 8

How can the difference in nobility be measured?

Answer 8

The difference in nobility can be measured as a difference in voltage potential.

Question 9

How is galvanic reaction related to batteries?

Answer 9

Galvanic reaction is the principle upon which batteries are based.

Question 10

What factors influence the severity of galvanic corrosion?

Answer 10

The severity of galvanic corrosion depends on the amount of separation in the galvanic series and the relative surface areas of the two metals.

Question 11

How does the relative surface area of the anode and cathode affect galvanic corrosion?

Answer 11

Severe corrosion occurs if the anode area (area eaten away) is smaller than the cathode area.

Question 12

What factors do not affect the severity of galvanic corrosion?

Answer 12

The severity of galvanic corrosion does not depend on the amount of contact, volume, or mass of the metals.

Question 13

What is a key prevention method for galvanic corrosion involving material selection?

Answer 13

To prevent galvanic corrosion, do not connect dissimilar metals. If unavoidable, electrically isolate one metal from the other using methods like rubber gaskets.

Question 14

How can adjusting the sizes of the anode and cathode help prevent galvanic corrosion?

Answer 14

Making the anode large and the cathode small can help reduce the severity of galvanic corrosion.

Question 15

What is an example of a bad material combination that could lead to severe galvanic corrosion?

Answer 15

Steel siding with aluminum fasteners is a bad combination that could lead to severe galvanic corrosion.

Question 16

What is an example of a better material combination to reduce galvanic corrosion?

Answer 16

Aluminum siding with steel fasteners is a better combination to reduce galvanic corrosion.

Question 17

How can eliminating the electrolyte help prevent galvanic corrosion?

Answer 17

Eliminating the electrolyte can prevent the electrochemical reactions necessary for galvanic corrosion to occur.

Question 18

What are two types of protection methods against galvanic corrosion?

Answer 18

Galvanic protection and anodic protection are two methods to prevent galvanic corrosion.

Question 19

What causes differential aeration corrosion?

Answer 19

Differential aeration corrosion is caused by varied oxygen concentrations leading to the oxidation of metal.

Question 20

Name three examples of corrosion caused by differential aeration.

Answer 20

Pitting corrosion, water line corrosion, and crevice corrosion.

Question 21

What happens to metal below the waterline in a metallic tank due to differential aeration?

Answer 21

The metal below the waterline is poorly oxygenated and acts as an anode, leading to corrosion.

Question 22

What role does metal above the waterline play in water line corrosion?

Answer 22

The metal above the waterline is highly oxygenated and acts as a cathode.

Question 23

Why can corrosion be accelerated in inaccessible places?

Answer 23

Oxygen-deficient areas serve as anodes, and cracks can focus corrosion.

Question 24

How do dirt, sand, scale, and other contamination accelerate corrosion?

Answer 24

They restrict oxygen access, establishing anodes and promoting greater accumulation of rust and scale.

Question 25

What is pitting corrosion?

Answer 25

Pitting corrosion is a form of extremely localized corrosion that creates small holes in the metal.

Question 26

List factors that can lead to pitting corrosion.

Answer 26

Damage to the protective film, scratches, local strain, chemical attack, surface roughness, and turbulent fluid flow.

Question 27

How can pitting corrosion be prevented?

Answer 27

Use alloys with high chromium, molybdenum, and nitrogen, ensure smooth surface finishes, remove contaminants, and avoid crevices in design.

Question 28

What causes corrosion in crevices?

Answer 28

Corrosion in crevices is due to oxygen deficiency and changes in acidity.

Question 29

How can crevice corrosion be prevented?

Answer 29

Control the environment to low chloride content and temperature, use high-resistance alloys, prepare smooth surfaces, and avoid crevice-forming designs.

Question 30

Why is corrosion more severe in crevices?

Answer 30

Crevices retain solutions that take longer to dry out, leading to more severe attack.

Question 31

What is stress corrosion?

Answer 31

Stress corrosion is the combined effect of static tensile stress and corrosive environments on a metal.

Question 32

What conditions are required for stress corrosion to occur?

Answer 32

The presence of tensile stress and a specific corrosive environment.

Question 33

Give examples of specific environments that cause stress corrosion in metals.

Answer 33

Caustic alkali and strong nitrate solution for mild steel, traces of ammonia for brass, and acid chloride solution for stainless steel.

Question 34

What are common sources of tensile stress in metals?

Answer 34

Deformation during fabrication, unequal cooling rates, internal stress rearrangement, and stress from rivets, bolts, and shrink fits.

Question 35

How can stress corrosion be prevented?

Answer 35

Eliminate high-stress areas through careful design and fabrication.