cathodic protection and prevention

Question 1

what is passive protection?

Answer 1

intentionally adding a metal to the circuit to supply electrons to the cathode

Question 2

what is active protection?

Answer 2

we supply electrons from a specially designed power supply system and controller, they are more complex but more efficient

Question 3

what kind of current do you use for cathodic protection?

Answer 3

DC

Question 4

how does an impressed current system work?

Answer 4

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.

Question 5

in an impressed current protection system what direction should the electrons flow?

Answer 5

toward the structure that is to be protected

Question 6

what are some potential disadvantages of impressed current protection systems?

Answer 6

they can “over protect” certain metals causing embrittlement, debonding the protective paint, more exposed cathode than the system can keep up with.

Question 7

what is a catastrophic side effect of an impressed current corrosion system failing?

Answer 7

stray current corrosion

Question 8

what are characteristics of a sacraficial anode system?

Answer 8

the metal has a corrosion potential more negative than metal, were trying to protect and its expendable

Question 9

what is aluminum’s corrosion potential?

Answer 9

-.76 to -1 volts

Question 10

why do aluminum anodes NOT develop the corrosion resistant film that most aluminum does?

Answer 10

because it it alloyed with small amounts of tin

Question 11

what are the shortcomings of using aluminum as an anode?

Answer 11

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.

Question 12

what is magnesiums corrosion potential?

Answer 12

1.60 to -1.63 volts

Question 13

what is the downfall of using magnesium as an anode?

Answer 13

it is too active, its high corrosion potentials create a current that can damage surrounding paint.

Question 14

where should you NOT use magnesium anodes?

Answer 14

salt water

Question 15

where do you find magnesium anodes?

Answer 15

protection of soil-burried structures, out board and inboard engines in fresh water, on board hot water heaters

Question 16

what is the corrosion potential of zinc?

Answer 16

-.98 to -1.03 volts

Question 17

what are some good metals to use under the waterline?

Answer 17

bronze, cupronickles, duplex stainless steel

Question 18

what are some good metals to use above the waterline?

Answer 18

stainless steel, marine grade aluminum, some inhibited brasses

Question 19

how many volts do we want to stay within in order to avoid a catastrophic anode cathode relationship?

Answer 19

.20 volts 200milivolts

Question 20

at what percentage humidity rate does corrosion go up drastically?

Answer 20

65%

Question 21

what things can we do to prevent water from collecting?

Answer 21

avoid upward angle brackets, eliminate pockets and dead ends, keep surfaces clean, avoid cracks and joints, grind down welds and beads, elminate weld beads

Question 22

what are the two major categories of plastics?

Answer 22

thermoplastics and thermosetting

Question 23

what are charateristics of thermoplastics?

Answer 23

materials that soften when heated and harden when cooled

Question 24

what are some examples of thermoplastics?

Answer 24

polyethelyne, polypropylene, nylon, PVC, some polyurethenes

Question 25

what are characteristics of thermosetting plastics?

Answer 25

they can not be softened when heated

Question 26

what are some examples of thermosetting plastics?

Answer 26

epoxies, polyesters, silicones, and some polyurethenes and rubbers

Question 27

what are polyurethane sealants known for?

Answer 27

their adhesive strength, they will cure under water and they have good chemical and moisture resistance. they are used for joints, portlight frames, and underwater thru-hull fittings.

Question 28

what are some examples of polyurethane?

Answer 28

3M-5200 and sika flex 420

Question 29

what are characteristics of poluysulfides?

Answer 29

similar to polyurethane without the permenant level strength. they can be used above and below the water line. it takes a little longer to cure and can not be used with ABS, PVC, acrylic, polycarbonate, or other plastics, as it will chemically attack them

Question 30

what are the two major groups of coatings?

Answer 30

mettalic and non metallic

Question 31

what are the two major groups of metallic caotings?

Answer 31

barrier coatings and sacrificial coatings.

Question 32

what are charactaristics of barrier coatings and what are some examples?

Answer 32

barrier coatings protect the substrate by preventing moisture from penetrating. they also provide protection from abrasion and wear by their hardness and lubricity. examples are chromium, nickle and tin.

Question 33

what are characteristics of sacraficial coatings?

Answer 33

they provide some degree of barrier coating but also act as a sacraficial anode. scratches are self healing in water.

Question 34

what is the basic rule for fastening dissimlar metals?

Answer 34

the fastening should be more nobel than the material being fastened.

Question 35

what thread types should not be used in hull structures?

Answer 35

rolled. instead use cut, because they fit more tightly

Question 36

what defines a machine screw?

Answer 36

bolts of a smaller diameter, less than 3/4”