NSTM CH 633 Flashcards
(18 cards)
What advantages do aluminum anodes offer, both physical & electrochemical?
Much lower consumption rate than magnesium, zinc, or iron, resulting in a 30% longer service life
Aluminum anodes weigh significantly less
Do not install over bare metal
Do not paint the anode
What is the half cell potential for? Zinc Aluminum Low Voltage Aluminum (Gallium Alloy) Magnesium
Zinc - Negative 1.04VDC
Aluminum - Negative 1.06V
Low Voltage Aluminum - Negative .8 to .85 VDC
Magnesium - Negative 1.55VDC
What "MIL" spec is each anode? Iron Low Voltage aluminum Zinc Aluminum Magnesium
Iron - MIL-A 19521 Low Voltage - MIL-A-24779 Zin- MIL-A-18001 Aluminum - MIL-A-24779 Magnesium - ASTM B43
For anodes what does their 3 letter designation mean
First
Second
Third
First - Anode Material
Second - Designates shape or general use
Third - Designates core
Should zinc anodes be installed over bare metal?
What is the preferred method of attachment?
When would Bolt-On anodes be used?
No, apply the surface coating system before the zinc anode is installed
Weld on, because of the secure electrical and mechanical attachment
Bolt-On is used if periodic maintenance and replacement is to be done without use of an industrial facility
Why were low voltage aluminum anodes (gallium) developed?
To provide cathodic protection to materials that are susceptible to hydrogen embroilment at zinc and aluminum operating potentials
Aluminum anodes on Navy Ships or Submarines conform to the requirements in ..
MIL-A-24779
How often does Sacrificial Cathodic Protection need replacement?
Impressed Current Cathodic Protection?
Approximately every 3 years (18-36months)
Service life of Components supports extended (12 years) dry-dock cycle.
What anode is effective in polarizing steel or aluminum surface?
Zinc Anodes
What are common anodes materials used to protect steel alloys?
Copper?
Steel- Magnesium, zinc, aluminum are common material
Copper- Iron
How is growth cleaned on an impressed current cathodic protection system anode?
By hand with a wooded or plastic scraper & white or green abrasive hand pads
Don’t damage the glass reinforced plastic (GPR) holder & the platinum coated wire rod
For anode attachment on steel surfaces what shall the studs and nut be fabricated from?
How do you install anode?
Medium or Mild Steel, Galvanized Steel or Corrosion Resistant Steel
- Apply anti-seize compound to studs
- Install 2 Star type lock-washers
- 1 between the nut and anode strap and 1 between the anode strap and the structure
Type ZEP anodes procured with a 3/8 inch NPS pope coupling core. How are they installed?
Can be installed by;
- Welding a 3/8 inch NPS pipe nipple to the attachment point & threading the anode on the pipe nipple
- Slip the pip coupling core over a 1/2 inch weld stud and bolting the anode in place
How do you attach an anode to an aluminum surface?
- With aluminum studs, CRES star type lock washer and CRES locknuts
- Apply anti-seize to threaded surfaces of the studs
When would magnesium anode be used?
Can you use in an enclosed space?
- Not in salt water
- Not on aluminum alloy surfaces
- Not in enclosed spaces without proper ventilation
- They require use of dielectric shields
- Use fresh water where the resistively is greater than 500 EM
No, not without proper ventilation
What are the 4 basic methods for controlling corrosion?
Corrosion Resistant Materials
Environmental Control
Coatings
Cathodic Protection
Tell me about anode placement on steel hulls
25 % stern anodes shall be bolted for ease of replacement.
60% of anodes installed amid ships
40% of anodes installed towards the stern
shall not be attached
to propellers, propeller shafts, rudders, diving planes, rudder skegs, strut arms, or on the exterior of the stern or
strut tube fairwaters.
Where is this anode installed?
ZEP?
ZHC?
ZTS?
ZHB?
ZEP=Stern Tube and Strut Bearing
ZHC=Stern area on steel hulls
ZTS= Aluminum hulls
ZHB=Wooden and Plastic GRP Surfaces
