Corrosion Prevention Flashcards
What are the four main things that can be done to prevent corrosion?
- Judicious design: informed choices on materials, geometry and configuration
- Protective coatings: passive metals (oxide films), paint, polymer coatings
- Corrosion inhibitors: chemicals added to the corrosive medium to thwart the rate of corrosion
- Monitoring: protective maintenance or regular replacement
How do you prevent galvanic corrosion?
- Avoid using dissimilar metals
- Electrically isolate one metal from the other if dissimilar
- Make the anode metal large comparison to the cathode metal
How do you prevent pitting and crevice corrosion?
- Watch for stagnate water or other electrolyte solution
- Use gaskets
- Use good welding practice
- change design to minimize microenvironment
How do you minimize Intergranular corrosion?
-monitor grain size, temperature, environment especially for stainless steel and aluminium
Name types of organic protective coating
Paint, ceramic & chrome. Danger of scratches!
Name types of cathodic protection?
- Zinc or galvanized coating on metals
- Sacrificial coating such a magnesium on steel boat hulls
- Impressed current
Name the three layers of a typical coating and their basic properties
- Top coat: sacrificial. Resistant to ion peritration and alkali. Insulator
- Intermediate: inhibitive. Low moisture and vapour transfer rate
- Primer: barrier, adheres to substrate, resists corrosion
Name the three groups of coatings
Metallic, organic and inorganic
Name the three methods of protection coatings can provide
Exclusion
Sacrificial Protection
Inhibition
What are inhibited coatings?
Inhibited coatings primarly acts as excluders but also provide a reservoir of corrosion inhibitor at the metal surface to protect against vapor penetration through coating .
Coatings include- paint priming, oils and greases that contain a corrosion inhibitor in suspension
What is exclusion coating?
Vitreous enamels, lacquers, non-inhibited paints,
various plastics, non-inhibited temporary protective
coatings (oil, grease etc.) and those metal coatings
that are more noble (cathodic) eg. Ni or Sn on steel,
than the base metal they are protecting.
All coatings will exclude the environment to some
extent but to protect by exclusion alone they must
cover the surface completely and resist mechanical
damage.
What is sacrificial coating and what are are the two ways of which it can act?
Sacrificial coating acts in two ways
• where coating is continuous they act as excluders
• where coating is discontinuous they provide cathodic
protection. In this case the coating is less cathodic than the base metal which remains protected when the sacrificial coating is damaged.
– Sacrificial coatings must be thick enough to last the life of the component they are protecting as they corrode away as they protect
Give some examples of sacrificial coating
Examples:
• Steels protected by Zn, Cd or Al coatings
• Copper protected by Zn coatings
• Aluminium alloys protected by pure Al or other less corrodible, AL alloys.
What are some types of metallic (commonly used) coating?
Electroplated coatings
Hot-dipped coatings
Cementation and oxide coatings
Sprayed coatings
What is electroplated coating?
– electrochemical process where metal ions are transferred from a solution and are deposited as a thin layer onto surface of a cathode.
– virtually all metals can be electroplated onto a variety of base metals
– coating thickness can be accurately controlled on plane surfaces but varies more on corners, inside recesses or hollow features
– electroplated coatings may protect by exclusion or by being sacrificial
– Eg. Zn and Cd are both anodic to steel and protect by being sacrificial. Sn and Ni are cathodic to steel so they must protect by being perfect excluders.
