1. BIMETALLIC CORROSION WORKSHOP
PRESENTATION
ON 01 – 02 JUN 2017

2. PRESENTATION 01: THE WEAKEST LINK.
PRESENTED BY:
AFRISUN COATING ENTERPRISES (PTY) LTD
PRESENTER: DANIEL VAN DER WESTHUIZEN
PRESENTATION 01: THE WEAKEST LINK

3. MOST SERIOUS THREAT TO ASSET OWNERS
THE WEAKEST LINK
The fate of a complex steel assembly may be determined by the vulnerability of its smallest and least considered components.
The bolts or fasteners holding the assembly together are often the areas where corrosion starts first, and where the effects of corrosion may have the most serious consequences
MOST SERIOUS THREAT TO ASSET OWNERS

4. WHY DO BOLTS RUST SO EASILY?
Looking at steel bolted systems, we find large components assembled and secured using relatively small nuts and bolts.
Such assemblies are found on pipelines, machinery, lattice towers and other metal infrastructure.
Wherever they are found — in the joins and most particularly in the bolts — is where the greatest corrosion effects will be found.

5. AT THE FASTENERS WHERE JOINT IS CLAMPED
ALL STRUCTURES MOVE
PIPELINES – Pulsation, Vibration & Thermal Fluctuation
MACHINERY – Pulsation, Vibration & Thermal Fluctuation
LATTICE STRUCTURES – Weight Loads, Thermal Fluctuation, Lateral Deflection (wind)
AT THE FASTENERS WHERE JOINT IS CLAMPED

6. BOLT CORROSION MECHANISMS
Differences in corrosion potential, relative size, mixed materials, damage to coatings due to structural “movement” and crevices formed between components affects bolt corrosion
Low-alloy steel flange connected to a high-alloy flange using low-alloy bolts and nuts

7. CROSS-SECTION OF CORRODED NUT & BOLT
BOLT CORROSION
CROSS-SECTION OF CORRODED NUT & BOLT

8. BOLT PROTECTION AGAINST CORROSION
APPLY CONVENTIONAL COATINGS FOR PROTECTION
HARD CURING COATINGS CRACK
ALLOWING MOISTURE & OXYGEN INGRESS

9. NUT & BOLT CORROSION

10. BOLT PROTECTION AGAINST CORROSION
HOT DIP GALVANIZING OR SHERARDIZING FASTENERS FOR PROTECTION
HOT DIP GALVANIZING THREADS OF THE NUT ARE OVERSIZED TO CREATE SPACE FOR THE ZINC
BY DOING SO THE TENSILE STRENGTH OF THE BOLT ARE WEAKEND

11. TENSILE STRESS VS. SHEAR STRESS

12. LOAD DISTRIBUTION The first two threads hold 57% of the load
If the first thread fails, the rest follow in a domino effect

13. TENSILE STRESS VS. SHEAR STRESS

14. TENSILE STRESS VS. SHEAR STRESS

15. TENSILE STRESS VS. SHEAR STRESS
Failure occurs in the threads at the fastener’s weakest point, made weaker by oversizing.
The amount of tension we can create in a bolt not only depends on the strength of its body but on the shear strength of its threads.
Thread pitch engagement needs to be close to 100%, or great enough to allow us to develop the full ultimate strength of the bolt.

16. BY DEFINITION - FASTENERS - BIMETALS
“Bimetallic corrosion occurs when 2 metals, with different potentials, are in electrical contact while immersed in an electrically conducting liquid. Because the metals have different natural potentials in a liquid, a current will flow from the anode (more electronegative metal) to the cathode (more electropositive metal), which will increase the corrosion on the anode.”
Corrosion Exclusively Vol.2 Issue 2, 2016 p26.

17. WHAT CAUSES CORROSION? There is moisture (electrolyte) There is oxygen
Multi-metals (fasteners, washers, flanges, welds)
ALLOWS THE FORMATION OF A CORROSION CELL

18. CORROSION CELL COMPONENTS: WATER (H2O) OXYGEN (O2)
ELECTRON FLOW (Electric current)
Iron (Fe)
SACRIFICIAL ANODE

19. RISKS ASSESSMENT: BY DEFINITION
FASTENERS - BIMETALS
NUT, BOLT, WASHER, MEMBERS
MOISTURE
OXYGEN
RESULT
BIMETALLIC CORROSION

20. ATHMOSPHERIC VAPOUR
PLANTS - DEW
METAL - CONDENSATE

21. DEW POINT TEMPERATURE
The dew point temperature is the temperature at which the air can no longer "hold" all of the water vapour which is mixed with it, and some of the water vapour must condense into liquid (water). The dew point is always lower than (or equal to) the air temperature.

22. CONDENSTATE RUNDOWN

23. FASTENERS

24. FLANGES (AIR CONDITIONING)

25. CAR RENTAL GEORGE AIRPORT

26. UNDER PIPE SUPPORT CORROSION

27. SURFACE PIPES

28. COATED SURFCE PIPES (Bimetals)
PIPES & UNDER PIPE SUPPORTS

29. MEMBER INTERFACES NUTS, WASHERS & BOLTS (Bimetals)

30. NUTS & BOLTS (Bimetals)
FLANGE METAL IS THE ANODE & FASTENER IS THE CATHODE

31. NUTS & BOLTS (Bimetals)
FLANGE METAL IS THE CATHODE & FASTENER IS THE ANODE

32. DECIDE ON HOW TO PROTECT YOUR ASSETS
FASTENERS
FLANGES
UNDER PIPE SUPPORTS
SURFACE PIPES
MEMBER INTERFACES
JOINTS (WELDS)

33. I THANK YOU. ARE THERE ANY QUESTIONS
I THANK YOU! ARE THERE ANY QUESTIONS? AFRISUN COATING ENTERPRISES (PTY) LTD /