cathodic protection and prevention Flashcards
(36 cards)
what is passive protection?
intentionally adding a metal to the circuit to supply electrons to the cathode
what is active protection?
we supply electrons from a specially designed power supply system and controller, they are more complex but more efficient
what kind of current do you use for cathodic protection?
DC
how does an impressed current system work?
an anode in electrolyte is connected to one side of the DC power supply, the metal to be protected is connected to the other side. the galvanic current between them is measured by a reference electrode, if unfavorable current flow is adjusted automatically by the power supply to compensate.
in an impressed current protection system what direction should the electrons flow?
toward the structure that is to be protected
what are some potential disadvantages of impressed current protection systems?
they can “over protect” certain metals causing embrittlement, debonding the protective paint, more exposed cathode than the system can keep up with.
what is a catastrophic side effect of an impressed current corrosion system failing?
stray current corrosion
what are characteristics of a sacraficial anode system?
the metal has a corrosion potential more negative than metal, were trying to protect and its expendable
what is aluminum’s corrosion potential?
-.76 to -1 volts
why do aluminum anodes NOT develop the corrosion resistant film that most aluminum does?
because it it alloyed with small amounts of tin
what are the shortcomings of using aluminum as an anode?
its susceptible to erosion in fast-moving water, it tends to suffer from pitting corrosion at elevated temperatures, and it is not as efficient as zinc.
what is magnesiums corrosion potential?
1.60 to -1.63 volts
what is the downfall of using magnesium as an anode?
it is too active, its high corrosion potentials create a current that can damage surrounding paint.
where should you NOT use magnesium anodes?
salt water
where do you find magnesium anodes?
protection of soil-burried structures, out board and inboard engines in fresh water, on board hot water heaters
what is the corrosion potential of zinc?
-.98 to -1.03 volts
what are some good metals to use under the waterline?
bronze, cupronickles, duplex stainless steel
what are some good metals to use above the waterline?
stainless steel, marine grade aluminum, some inhibited brasses
how many volts do we want to stay within in order to avoid a catastrophic anode cathode relationship?
.20 volts 200milivolts
at what percentage humidity rate does corrosion go up drastically?
65%
what things can we do to prevent water from collecting?
avoid upward angle brackets, eliminate pockets and dead ends, keep surfaces clean, avoid cracks and joints, grind down welds and beads, elminate weld beads
what are the two major categories of plastics?
thermoplastics and thermosetting
what are charateristics of thermoplastics?
materials that soften when heated and harden when cooled
what are some examples of thermoplastics?
polyethelyne, polypropylene, nylon, PVC, some polyurethenes