BIMETALLIC CORROSION WORKSHOP PRESENTATION ON 01 02 JUN

BIMETALLIC CORROSION WORKSHOP PRESENTATION ON 01 – 02 JUN 2017

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

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

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.

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

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

BOLT CORROSION CROSS-SECTION OF CORRODED NUT & BOLT

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

NUT & BOLT CORROSION

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

TENSILE STRESS VS. SHEAR STRESS

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

TENSILE STRESS VS. SHEAR STRESS

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.

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 p 26.

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

CORROSION CELL COMPONENTS: WATER (H 2 O) OXYGEN (O 2) ELECTRON FLOW (Electric current) Iron (Fe) SACRIFICIAL ANODE

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

ATHMOSPHERIC VAPOUR PLANTS - DEW METAL - CONDENSATE

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.