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