3.5% INDUSTRIALISED GDP ON CORROSION. Corrosion is degradation in properties or appeared. Caused by dissolution and surface oxidation
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What are direct and indirect corrosion losses
Direct loss - component replacement, use of expensive corrosion resistant materials, painting. Indirect loss - plants shut down, loss of product in pipes, loss of efficiency, contamination
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What effect is corrosion?
Corrosion is electrochemical effect. Metals tend to form oxides in oxidation,
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What does reduction gain?
Reduction is gain of electrons, oxidation is loss
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What is general process of corrosion?
Metal atoms are oxidised and give up electrons. This occurs at anode. Electrons transferred to other chemical species at cathode
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How does rust form for iron?
At anode, iron loses two electrons. O2+2H2O + 4e- -> 4OH- at cathode. Precipitation: Fe2+ + 2OH- -> Fe(OH)2 Rust: 4Fe(OH)2+O2-> 2Fe2O3H2O+2H2O
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Why does corrosion continue to increase?
As energy of oxidation increases electrochemical cell action, corrosion increases
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What is Galvanic series?
Galvanic series - inert materials more cathodic, reactive materials more anodic.
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What are ways to combat corrosion?
Use materials resistant to corrosion (usually cannot), use paint barrier, or galvanic protection/galvanising - metal to be protected connected to more reactive one like Mg or Zn, cathodic protection - reverses oxidation
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How does paint coating work? Why is it not good?
HDPE top coat provides stress cracking resistance, indentation resistance, abrasion resistance, and flexibility. then HDPE intercoat and epoxy primer for adhesion and chemical resistance. If paint film damaged, steel rust builds
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How are magnesium rods used to protect underground steel pipes?
Connecting steel to magnesium makes iron-magnesium voltaic cell. Magnesium more reactive so it is oxidised. Magnesium rod easier to replace than digging up steel pipe. Mg anode, Fe cathode. Soil electrolyte
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How does hot dripping protect steel?
Hot dripping zinc makes galvanised coating. Corrosion of zinc produces precipitate which protects steel. Zinc takes long time to weather, coating preferred to corrode, and protection from products protects steel
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What are galvanised steel benefits?
Coating is bonded metallurgically, long life, cheap, life over 40 years
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How does cathodic protection work?
Cathodic protection redirects current from sacrificial anode to protective anode. Electrons flow to protected metal, cathode, and protective anode sacrifices electrons
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Other cards in this set
Card 2
Front
What are direct and indirect corrosion losses
Back
Direct loss - component replacement, use of expensive corrosion resistant materials, painting. Indirect loss - plants shut down, loss of product in pipes, loss of efficiency, contamination
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