Metallic Coatings Flashcards
How do coatings provide protection?
- Exclusion (barrier)
- Sacrificial
- Inhibition
Key steps in Hot dip Galvanizing
1) Substrate
2) Welding
3) Cleaning
4) Annealing
5) Bath entry
6) Bath
7) Bath Exit
8) Tempering
9) Inspection
Substrate (Hot Dip Process)
- Chemistry of the steel substrate is determined by customer demands
- Galvinizing process is completed after hot and cold rolling
Welding ( Hot Dip Process)
-Welding coils allows the process to be continuous, the back end of one coil and the front end of another
Cleaning (Hot Dip Process)
- 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
Annealing (Hot Dip Process)
- 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
Bath entry (Hot Dip Process)
- 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
Bath (Hot dip Process)
- 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)
Bath Exit (Hot dip process)
- 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
Temper Mill (Hot Dip Process)
- 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)
Sources of Defects in Hot Dip Process
-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)
Cleaning Defects
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
Bath Entry Defects
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
Bath Defects
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
Bath Exit Defects
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)
Temper Mill Defects
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
Physical Vapour Deposition steps
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
Zinc based coatings
Galvanic protection from more negative standard reduction potential AND barrier protection from insoluble zinc hydroxide layer
Aluminium based coatings
- 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
Three types zinc coating
- Galfan
- Galvalume
- Galvanneal
Electroplating
• 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
Steps of elctroplating
- Entry
- Cleaning
- Pickling
- Plating
- Drag out of tank
- Oiling
- Visual Inspection
Entry (Electroplating process)
-Coils welded together.
Accumulator used to ensure process is continuous. A tension leveller corrects
for shape.
Cleaning (Electroplating process)
-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.
Pickling (Electroplating process)
- 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
Plating (Electroplating process)
• 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
Important Bath parameters associated with Electroplating
1) Type and conc of electrolyte (Additives)
2) Temperature
3) Current density
4) Shap and Distance between anode and cathodes
5) Bath Agitation
Drag out of Tank (electroplating process)
- 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.
Oiling (Electroplating process)
- minimizes damage, helps lacquering and printing processes
* two main methods; electrostatic oiling and emulsion oiling.
Visual Inspection (Electroplating process)
Visual inspection and monitoring of tin coating weight, thickness and presence of pinholes.
Electro-less plating
• 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.
