Effects of Hot Dip Galvanizing on High Tensile Fasteners

Effects & Side effects of Hot Dip Galvanizing on fasteners

And alternative corrosion protection

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.

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

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.

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

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.

What are alternatives

• Alternatives: – Using uncoated Fasteners

– Using flZnnc Fasteners

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.

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

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.

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

Thank You

Contact: abdullahta@gmail.com