6 - Corrosion Prevention Techniques Flashcards
Excellent corrosion and high
temperature resistance. Brittle and has lower tensile
strength
Ceramics
Weaker, softer, and more
resistant to chloride ions but
less resistant to strong sulfuric
acid and oxidizing acids
Rubbers and Plastics
Good corrosion resistance,
electric and heat conductivity. Fragile
Carbon
Corrosion resistance of pure metal =
better vs corrosion resistance of metal
with impurities
METAL PURIFICATION
Not expensive in a fairly pure state
(99.5% pure). Used for handling hydrogen peroxide
Aluminum
Generic name for a series of more than
30 different alloys. Does not resist all corrosives. Natural metal-corrosive combination.
Stainless steel
More corrosion resistant than
induction-melted zirconium. Used in atomic-energy application
Arc-melted Zirconium
A technique to control corrosion of a
metal surface by making it the cathode
of an electrochemical cell
CATHODIC PROTECTION
He said copper could be successfully protected
against corrosion by coupling it to iron
or zinc in 1824
Humphry Davy
Uses a more reactive metal (sacrificial
anode) that corrodes preferentially to
the metal structure (cathode) that
needs to be protected
Galvanic or Sacrificial Anode
Method
CATHODIC PROTECTION ADVANTAGES
- No external power source required so it
can be used remotely - Lower installation cost
- Minimum maintenance is required
- Systems seldom cause adverse effects
on their structure
CATHODIC PROTECTION DISADVANTAGES
- Limited driving potential and current
input - Can be ineffective in high resistivity
environments - Not generally applicable for poorly
coated structures - Life of anodes are relatively short
depending on the load - Existence can easily be forgotten by
maintenance personnel
External power supply is connected to
an underground tank. The negative
terminal of the power supply is
connected to the tank, and the positive
to an inert anode such as graphite or
Duriron.
Impressed Current Method
Impressed Current Method ADVANTAGES
- There is a larger driving voltage
- More flexible control of voltage and
current - Applicable to uncoated parts, high
resistivity environments, large objects,
etc.
Impressed Current Method DISADVANTAGES
- Higher installation and maintenance
costs for small systems - Can cause adverse effects on nearby
structures
Aggressive corrosives require prohibitively high currents;
lower currents are needed to protect
steel in less severe environments. What is this called?
PROTECTIVE CURRENTS
FACTORS THAT INFLUENCE CURRENT
REQUIREMENTS
- Environment
- Coating
Achieved by exhibiting an active
passive behavior to form a passive film
ANODIC PROTECTION
Practical application and use of
potentiostat was first suggested by
Edeleanu
ANODIC PROTECTION ADVANTAGES
- Applicable in extremely corrosive
environments - Low current requirements
ANODIC PROTECTION DISADVANTAGES
- Only applicable to passive metals and
alloys
Electronic device that maintains a
metal at a constant potential with
respect to a reference electrode in
order to anodically protect a structure
POTENTIOSTAT
What are the terminals of Potentiostat?
o Connected to the tank,
o Auxiliary cathode,
o Reference electrode
Can be accurately and rapidly determined by electrochemical measurements
Anodic protection
Must usually be determined by empirical testing
Cathodic protection
Range of corrosive is from weak to aggressive
Anodic protection
Range of corrosive is from weak to moderate
Cathodic protection
