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Coating Formulations > Metallic Coatings > Flashcards

Metallic Coatings Flashcards

1
Q

How do coatings provide protection?

A
  • Exclusion (barrier)
  • Sacrificial
  • Inhibition
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2
Q

Key steps in Hot dip Galvanizing

A

1) Substrate
2) Welding
3) Cleaning
4) Annealing
5) Bath entry
6) Bath
7) Bath Exit
8) Tempering
9) Inspection

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3
Q

Substrate (Hot Dip Process)

A
  • Chemistry of the steel substrate is determined by customer demands
  • Galvinizing process is completed after hot and cold rolling
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4
Q

Welding ( Hot Dip Process)

A

-Welding coils allows the process to be continuous, the back end of one coil and the front end of another

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5
Q

Cleaning (Hot Dip Process)

A
  • Enhance coating adherence and avoid bath contaminations
  • Chemical Cleaning (Spray bars supply heated 3% NaOH which removes residual contamination and squeegee rolls used to minimise carry-over
  • Mechanical (Polypropylene brushes supplied with NaOH to ensure cleanliness)
  • Substrate passes through secondary set of squeegee rolls, warm water spray, more squeegee rolls and dried
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6
Q

Annealing (Hot Dip Process)

A
  • DFF and radiant tube furnace used to preheat strip upon entry
  • Atmosphere modified into a reducing atm (H2 and N2) in lower parts of DFF
  • U/W shaped radiant tubes used to provide heat needed for annealing
  • Annealing takes place at above 700C
  • Rapidly quenched to temp of 470C at rates of 50C/s
  • Line speed control important and is changed in line with the amount of recrystallization required
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7
Q

Bath entry (Hot Dip Process)

A
  • The Snout is used to prevent oxygen from entering the reducing atm and controlling the production of zinc vapour
  • Atm within snout composed of HNx gas (Hydrogen component maintains a reducing atm, and Nitrogen is able to expel any residual oxygen)
  • Dry HNx gas supplied at the top of snout whilst that supplied closer to the molten zinc is humidified to prevent zinc vaporization
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8
Q

Bath (Hot dip Process)

A
  • Several reactions take place depending on bath conditions
  • Temp dependant on coating comp.
  • Al added to bath because it improves adhesion of produced coating, improves brightness and formability and inhibits the reaction between strip and bath and aids top dross formation
  • Al can exist as IMs and dissolved in liquid Zn
  • Dissolved Al inhibits Fe-Zn reactions during galvinizing
  • IMs can either float (Top dross) or sink (bottom dross)
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9
Q

Bath Exit (Hot dip process)

A
  • Air Knives used to force excess liquid back into the pot
  • Increased line speed increases the amount of zinc dragged out
  • Xray measurement used to measure uniform coating thickness
  • Knives blow inert Nitrogen
  • Height and pressure important to determine the nature of the dross formed at the surface. To reduce dross pick up a clear area around the emerging strip is required
  • If Knives are too high the cooling rate will be too slow and coating will not solidify
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10
Q

Temper Mill (Hot Dip Process)

A
  • Temper rolling enhances the mechanical properties of the coated product by suppressing yield point elongation
  • Work roll texture transferred to strip resulting in well defined surface topography
  • Finishing steps include application of pre-treatment,organic coatings, oiling, cutting and slitting
  • Then Inspection (Both automatic and manual)
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11
Q

Sources of Defects in Hot Dip Process

A

-Cleaning (Small irregular spots, Bare spots)
-Bath entry (Flaking, Needle defects)
-Bath (Dross, Dewetting)
-Bath Exit (Knife lines, coating sagging, Black spots)
Temper mill (Compact zinc, Microfolding)

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12
Q

Cleaning Defects

A

small irregular spots
-Incomplete removal of emulsions (high levels of iron fines increase likelihood of this)
Bare spots
-Small areas where zinc is missing, oil, grease introduced prior to furnace which not been totally burnt off
-Prevent zinc adhering
-Can be single or repetitive

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13
Q

Bath Entry Defects

A

Flaking
-Irregular patches where steel can be seen with peeling of zinc coating
-Poor adhesion, not correct conditions, Temp and H levels, oxidation results in zinc not adhering
Needle Defect
-Elongated feature
-Lack of suppression of zinc dust inadequate control of wet HNx gas
-Dust particles get dragged through the pot

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14
Q

Bath Defects

A

Dross
-IM compounds dragged out of bath incorporated into coating
Dewetting
-Streaky irregular pattern which consists of tiny bare spots
-Thinned coating in vicinity of bare spot
-HSLA, due to segregation of alloying elements

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15
Q

Bath Exit Defects

A

Knife Lines
-Dark/Thicker, zinc trapped in knife slot, disturbance of Nitrogen flow
Jet Lines
-light lines
-consequence of turbulent flow
-knife to strip distance to large
Coating sagging
-Knives are too high, cooling rate too slow, skin forms on liquid zinc and sags under its own weight
Cooling defect/Black spots
-Contact with the rools and cleanliness issues (Zn dust)

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16
Q

Temper Mill Defects

A

Compact Dust
-Tiny raised particles
-Build up of debris from temper mill accumulating on exit accumulator
Micro-Folding
-Suggest significant turbulence of liquid zinc due to some form of contamination

17
Q

Physical Vapour Deposition steps

A

1) Vaporisation from a solid source using a high temp vacuum (thermal evaporation) or gaseous plasma (sputtering)
2) Transportation to the substrate
3) Condensation onto the substrate resulting in creation of a solid thin film

  • Produces very uniform layers and coating thickness can be tightly controlled
  • single, multi or alloy layer
  • all metals can be used
  • energy expensive
18
Q

Zinc based coatings

A

Galvanic protection from more negative standard reduction potential AND barrier protection from insoluble zinc hydroxide layer

19
Q

Aluminium based coatings

A
  • Inert passive layer of Al2O3
  • Thickness of oxide layer can be increased by chemical treatment or app of current
  • Excellent heat resistance used in exhaust systems
  • Used where chiping is unlikely due to little galvanic protection
  • High MP means that Al and steel subtrate react to form thick IM coating
  • Si added to minimise the thickness of IM and improve formability
20
Q

Three types zinc coating

A
  • Galfan
  • Galvalume
  • Galvanneal
21
Q

Electroplating

A

• Shaped objects and continuous line
coating of sheet material.
• Superior finish to hot dipping
• Coatings can be painted easily
• Allows for differential coating weights
-Coating metal made the anode and substate metal cathode

22
Q

Steps of elctroplating

A
  • Entry
  • Cleaning
  • Pickling
  • Plating
  • Drag out of tank
  • Oiling
  • Visual Inspection
23
Q

Entry (Electroplating process)

A

-Coils welded together.
Accumulator used to ensure process is continuous. A tension leveller corrects
for shape.

24
Q

Cleaning (Electroplating process)

A

-Degreasing with alkaline solution (NaOH).
-The temperature is increased to 80°C to speed the process up.
-Rinsing with high pressure water.
-The use of shorter times can also be assisted by the use of electrolytic cleaning (anodic or cathodic or both).
-Controlled temperature,
cleaner concentration. Water rinse.

25
Q

Pickling (Electroplating process)

A
  • Light oxides will form so acidic picking dissolves oxides
  • Tin deposition will not occur on top of oxides
  • Can be done by immersion, spraying, or electrolytically
  • Process also lightly etches the strip surface to provide an active surface for plating
  • Anodic pickling; Iron dissolved and surface is etched
  • Cathodic pickling; Hydrogen evolution, reduce oxides off
26
Q

Plating (Electroplating process)

A

• Number of vertical plating tanks with the coating getting progressively thicker. The tanks are lined with rubber.
• Conductor rolls at top of tank
• Sink rolls at the bottom of the tank.
• Anodes hang from bridges at the top of the tanks.
• The number of anodes and inter anode spacing are adjusted
-An anode bridge and toe
guide accounts for voltage
drop between top and
bottom of tank

27
Q

Important Bath parameters associated with Electroplating

A

1) Type and conc of electrolyte (Additives)
2) Temperature
3) Current density
4) Shap and Distance between anode and cathodes
5) Bath Agitation

28
Q

Drag out of Tank (electroplating process)

A
  • minimises the loss of plating electrolyte carried over

* The strip is washed by passing it through a tank or by spraying it onto the steel.

29
Q

Oiling (Electroplating process)

A
  • minimizes damage, helps lacquering and printing processes

* two main methods; electrostatic oiling and emulsion oiling.

30
Q

Visual Inspection (Electroplating process)

A

Visual inspection and monitoring of tin coating weight, thickness and presence of pinholes.

31
Q

Electro-less plating

A

• it does not use power and involves the use of simple
equipment.
• non-conductive materials can be coated
• coatings are typically uniform with fewer pinholes
• non uniform geometries can be coated
• wear resistance and lubricity
• For electro-less plating to occur the reducing agent must
have a more negative potential than the metal to be plated.
• One of the most common types of electro-less plating is
nickel plating.
• The reducing agent (commonly hypophosphite) is used
in the electrolyte which contains a soluble salt of the
coating metal.

Coating Formulations (2 decks)

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