Effects & Side effects of Hot Dip Galvanizing (tZn) on fasteners and alternative corrosion protection

1. Effects of Hot Dip Galvanizing
on High Tensile Fasteners
Effects & Side effects of Hot Dip Galvanizing on fasteners
And alternative corrosion protection

2. Hot Dip Galvanizing business case
• Business Case of Hot dip galvanized fasteners
– Provided prolonged corrosion protection
– Easy sourcing being HDG not licensed proprietary coating
– Less cycle time compare to other corrosion protection
– Cost effective proven technology
– Does it mean, all good with Hot Dip Galvanized (tZn) fasteners?
• NO
– Why?
• Because there are number of factors which affect.
• If not taken due care, it poses a serious thread to structure.

3. Factors playing role in HDG Fasteners
• Process
– Acid pickling is most common step in 7 tank cleaning process.
– Acid pickling is main source of Hydrogen getting induced in fasteners
thru metal surface
– Any minor process non-compliance at acid picking & follow up baking
will cause Hydrogen embrittlement
– All high tensile fasteners going for hot dip galvanizing face this risk so
the purchaser has limited proven sources.
– Chances of re-tempering taking place while parts in HDG (tZn) bath

4. Factors playing role in HDG Fasteners
• Product
– Nut internal threads are machined after HDG (tZn) & does not posses
coating
– Nut are generally pre-lubricated with MoS2
• This MoS2 lubricant has its own life, if fasteners kept in stock lubricant
degrades.
– In service life time the nut lubricant loses its properties & then the
issues occur such as seizer, rust, connection preload failure etc.
– If Bolt & Nut assembly used then bolt is generally 6g+tZn & Nut
threads are oversized
– If bolt to be used with threads machined in component then bolt
threads to be undersized to a proper clearance
– Required specific CoF between Nut & Bolt threads.

5. Issues with Hot Dip Galvanized (tZn) Fasteners
• Long stored fasteners
– Issues will be very less with bolts, since having 100% zinc coating
– Nut lubricant will lose its properties to generate required without any
signs of corrosion.
– Long storage in open will also cause rust on nut threads
• Result of such fastener usage
– Thread washout on Nut or Bolt
– Torque will reach with out preloading bolt, in turn bolt will fail in
fatigue
– Hence Risk

6. Issues with Hot Dip Galvanized (tZn) Fasteners
• Assembled fasteners
– Nut lubricant will lose it properties over time after assembly.
• Result of such fastener continued usage without lubrication
– In service life time such joints loses preload
– While applying torque on nut during periodic maintenance complete
bolt assembly will rotate.
– If Torque applied by locking bolt head, Torque will reach with out
preloading bolt, in turn bolt will fail in fatigue if no thread damage
occurs
– Hence Risk.

7. What are alternatives
• Alternatives:
– Using uncoated Fasteners
– Using flZnnc Fasteners

8. Alternative to tZn : Uncoated Fasteners
• How to use uncoated fasteners?
– Fasteners must be oil hardened & oil tempered with minimum or no scale
– Bolt would be 6g & Nut of 6H – standard pairing.
– Apply moleykote paste on threads to have required CoF
– After assembly apply spray of metallic wax for corrosion protection
• Benefits
– No risk of hydrogen embrittlement
– No need of different bolts for use in machined threads & with nuts,
inventory control
– Coming from oil tempering, least possibilities of rusting
– Easily serviceable
• What is not Good?
– Metallic wax coat spray may cause stains around connection, causing
aesthetics look poor.

9. Alternative to tZn : flZnnc coated fasteners
• What is flZnnc coating?
– There are various proprietary coatings, organic & inorganic
– Coating containing Hexavalent Chrome are banned in Europe & US
– Generally flZnnc are without Hexavalent Chrome
– flZnnc coatings containing Zinc & Aluminum flakes.
– Can be applied over parts by Dip spin or spray based on size & shape
of parts
• Benefits
– Used widely for fasteners & provide long corrosion protection
– No need to have special tolerances for nut & bolt threads as like tZn
– Nut threads too are coated
– With suitable MoS2 past, CoF can be easily stabilized

10. In a Nutshell
• tZn coating have many benefits however also has various side
effects, it can cause serious issues in service life time
• Use of alternatives
– Usage of uncoated fasteners is one of the best cost effective
engineering solution however aesthetics to be given up to some
extent due to wax coat after assembly.
– Usage of flZnnc fasteners is another best solution without sacrificing
aesthetics however it may slightly increase the cost
• Conclusion
– It is important to consider lessons learnt, feasibilities, inventory
management & service life time cost effectiveness & then only decide
what is most appropriate selection of fasteners.

11. Something related
• Tightening methods
– Many times thread failure can be eliminated by choosing appropriate
tightening method
– Torquing
• This is conventional method where torque is applied by torque wrench on bolt
head or nut
• If lubricant is not good or expired, CoF rises & torque attainment occures
much before bolt preloading
• In this lubricant must be proven one & very criticle if tZn fasteners are involved
• Most of the connection failures find RCA to point finger at thread lubrication
– Tensioning
• This is modern technique to preload bolt/stud where studs is stretched with
known tensioning load & nut is simply tightened by normal rotations
• Lubricant play role of anti-seizer however will not cause any pre-matured
torque attainment
• Most proven & guaranteed tightening method to avoid loss of fastener preload
• Cost will increase slightly because in certain places instead of bolts, studs to be
used

12. Thank You
Contact: abdullahta@gmail.com