material selection for roof
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Material Selection for Roof
Sheeting
INTRODUCTION
• Investigation into corrosion of Rainbow Chicken Farms roof plates done previously showed that premature failing (2 years) of IBR pure zinc galvanised roof sheeting
• Due to severe attack by condensate on inside surface • Condensate high in sulphur: 3200mg/l • Recommended that condensate be controlled and more corrosion resistance roof be
installed
Objectives • One Eighty to select suitable materials for replacement of roof plates Investigation • A product sample was chemically analysed; material selection was conducted for offering
more resistance to condensate
RESULTS: COMPARISON OF
SULPHUR LEVELS Table 1: Comparison of Sulphur Levels
Sulphur Stream Sulphur Level [mg/l]
Rainbow Chicken Product (current product) 1096
Rainbow Chicken Product (previous result) 3200
RESULTS: FULL PRODUCT
ANALYSIS
Product Target Range for Industrial Water
Cl (Chlorine) 0-500 mg/l
Fe (Iron) 0.0-10.0 mg/l
pH 5-10
SO4 (Sulphates) 0-500 mg/l
TDS (Total Dissolved Solids) 0-1600 mg/l
CaCO3 mg/l 0-1200 mg/l
RESULTS: HOT DIP
COATINGS Coating Type Description Roof sheeting Application
Zinc Coating
(Galvanised)
Most commonly used coatings Used for roof sheeting applications, entry
level product
Zinc Iron Coating
(Galvanneal)
Usually used in the painted condition; has
excellent abrasion resistance
This coating and is used for autobody
panel, stairs and walkways rather than
roof sheeting
Aluminium Type 1 Coating Type 1 coatings are usually up to 30 µm thick
and are an aluminium silicon alloy
Used for automotive applications for
exhausts rather than roof sheeting
Aluminium Type 2 Coating Type 2 coatings are usually 30-50µm thick;
excellent corrosion resistance but can be poor in
salt water; rusting of cut edges in industrial and
rural environments
Used for roof sheeting applications
Zinc – 55 Al Alloy Coating
(Galvalume, Zincalume)
Zinc coating with additions of aluminium; has
excellent high temperature resistance
Used for industrial roof sheeting
applications.
Zinc – 5Al Alloy Coating
(Galfan)
Zinc coating with additions of aluminium; more
corrosion resistant than zinc coatings in severe
marine environments only
Used for roof sheeting applications
Pb-Sn Coating
(Terne)
Lead-tin alloy Used in automotive industry
RESULTS
RESULTS
RESULTS Coating
Designation
Minimum requirement
total both sides (g/m2)
Nominal Thickness per
side (µm)
Galvanised:
Z120
Z180
Z275
120
180
275
8.5
12.7
19.4
Galfan:
GF135
GF180
GF275
135
180
275
9.8
13.3
19.8
Galvalume:
AZ150
AZ165
AZ180
150
165
180
20.4
22.5
24.5
RESULTS
Figure 5: Attachment zone
Figure 6: Pitting
Figure 7: Corrosion of internal surface
RESULTS
Figure 8: Formicary corrosion- Internal and external surface
Figure 9: Formicary corrosion- External surface
RESULTS
Figure 10: Formicary corrosion- Internal surface
Figure 12: Perforated fin
Figure 11: Pitting of aluminium fin
RESULTS: COMPOSITIONAL
ANALYSIS Table 1: Results of compositional analysis (wt%)
Element Aluminium (Al) Silicon (Si) Sulphur (S) Copper (Cu)
Paint 92.22 – 94.77 0.00 – 7.78 0.00 – 5.23 -
Copper tube: Dark band 1 68.89 – 71.85 - 10.9 – 11.47 14.15 – 16.59
Copper tube: Dark band 2 9.98 – 15.52 - 2.41 – 3.05 81.43 – 87.80
Copper tube: Light band - - - 100
Pit – External surface - - - 100
Pit – Internal surface - - 0.00 – 0.92 99.08 - 100
Green Deposit on tube 81.38 – 80.87 - 6.72 – 7.29 11.83 – 11.9
Blue-Green crystals - - 35.33 – 75.08 24.92 – 64.69
RESULTS: IR TESTING
CONCLUSIONS • Corrosion of evaporator coil occurred by dual mechanisms of galvanic corrosion of aluminium fins
and internal and external formicary corrosion of copper tubes
• Ultimate failure: Aggressive formicary corrosion
• Vinegar source of organic acid for formicary corrosion of external surface
• High sulphur environment exacerbated pitting of copper tubes- formation of green crystals of copper
carbonate
• Further investigation was necessary to conclusively determine source of formicary corrosion
• Formicary corrosion caused widespread interconnecting pits: perforating copper tube, hence leakage
in two years
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