HOT DIP GALVANIZING AND DUPLEX COATING SYSTEMS…

• Different Types of hot dip galvanizing

•The general galvanizing process & Standard

•What determines ctg thickness & appearance

• Durability of HDG in the environment

• When selecting “galvanizing” specify correctly!

• Some strengths & characteristics of HDG.

• Repairs to HDG.

• Duplex coating systems.

Ductile - smooth - attractive appearance

Coating thickness 20 microns (Z275)

Fe/Zn alloys virtually absent

SPECIFICATION - SANS 4998/3575

Ductile - smooth - attractive appearance

Coating thickness 30 microns (SANS 675)

Fe/Zn alloys < 10% of coating thickness

SPECIFICATION - SANS 675/10244/935

Ductility diminished

Coating thickness > 85 microns

Greater corrosion resistance

SPECIFICATIONS – SANS 121 (ISO 1461) /

SANS 32 (EN 10240)

ZINC BATH

COOLING

AND

INSPECTION

CENTRIFUGING

NOT CONTROLLED BY

GALVANIZER (BIG

EFFECT)

Steel composition (Si & P)

Steel thickness

Surface roughness

CONTROLLED BY GALVANIZER (SMALL EFFECT)

Zinc temperature

Immersion time

Withdrawal rate

Angle of exit

Efficiency of centrifuge

COATING THICKNESS

IS RELATIVELY SELF

CONTROLLING

CA

ST

ING

SMINIMUM COATING THICKNESS ON ARTICLES THAT

ARE NOT CENTRIFUGED

SANS 121 (ISO 1461)

Category and

thickness (t)

SABS 763 (OLD)

Local

coating

thickness

µm

Mean

coating

thickness

µm

Article type

Thickness,

µm General

Application

t˃6 70 85A1,B1,D1,

E1,F185

3˃t≤6 55 70 A2,B2,F2 65

PR

OFI

LES

1.5≥t≤3 45

45

55

t1.5 35 A3, B3, F3 45, 55, 45

t6 70 80 D1 85

t6 60 70

• Coating thickness on structural

steel achieved varies from the

minimum upwards of 100%

greater.

• Life expectation predictions are

based on the minimum coating

thickness and are therefore

generally quite conservative.

Class Description

Corrosion

rate (av. loss

for steel

µm/y)

Corrosion

rate (av.

loss for Zn

µm/y)

* Typical life

in yrs to first

maintenance )

(Z275)

** Typical life

in yrs to first

maintenance

(70µm HDG)

C1 Interior: dry ≤ 1.3 ≤ 0.1 >50 >50

C2

Interior: occasional

condensation

Exterior: exposed rural

inland

> 1.3 to 25 0.1 to 0.5 >50 >50

C3

Interior: high humidity,

some air pollution

Exterior: urban inland or

mild coastal

> 25 to 50 0.5 to 2 10 to 40 35 to >50

C4

Interior: swimming pools,

chemical plant, etc.

Exterior: industrial inland

or urban coastal

2 to 4 5 to 10 18 to 35

C5Exterior: industrial with

high humidity or high

salinity coastal

>80 to 200 4 to 8 3 to 5 9 to 18

>50 to 80

• Nickel

• Zinc

• Carbon Steel, rolled

Ele

ctr

op

os

itiv

e

Mo

re c

ath

od

ic m

eta

ls

Ele

ctr

on

eg

ati

ve

Mo

re a

no

dic

me

tals

• Steel, low-alloyed

• Brass

• Cast Iron, unalloyed

• Cadmium

• Aluminium

• Magnesium

• Gold

• Silver

• Stainless Steel (18/8)

• Aluminium Bronze

• Copper

• Graphite

• Milnerton Chevron

Refinery (25+25yrs)

• Waterfront (15+15)

• Newlnds Rugby Grds –

Railway Stand (32+?).

• Capricrn Prk (Muizbrg)

•(3 & 25yrs)

Somrset Wst belw Sir LP

Truscon twrs (40yrs +?)

• Mitchells Plain TI (Lite

poles 19 to 32 yrs)

• Baden Powell Drive (10

to 12 yrs)

SPECIFIC CASE HISTORIES

SANS 3575/4998 (Z275)

SANS 121

Surface preparation determines coating

success. An “honest coating”

Adhesion - metallurgically bonded

Process is self controlling - law of

metallurgy determines coating thickness

Complete internal & external protection

Coating solidifies immediately on

withdrawal, quicker delivery.

Ease of inspection

60 to 80µm

• Zinc thermal spraying

• Zinc rich paint or epoxy• SANS 121 – Individual 10cm²

• SANS 121 – Total 0,5%

• Coating thickness or DFT 100µm

Use only as a mist coat over

appropriate repair material for

immediate aesthetical appeal.

Duplex provides a synergistic performance,

i.e.

• Duplex life = X (zinc life + paint life)

• Aggressive Marine X = 1,5

• 1,5 (10 years + 5 years) = 23 years

• Mild rural X = 2,7

• 2,7 (30 years + 15 years) = 120 years

Zinc has in the past been associated with lead as a heavy metal and therefore bad for the environment? The true facts:

– Zinc is not a heavy metal!

– There are 300 enzymes in ones body that that require zinc to function properly

– 48% of the worlds population are zinc deficient

– Zinc is used in ointments for combating sun burn, nappy rash, ADHD, etc.

– Used in time the throat lozenge can prevent a cold

– It is the aphrodisiac mineral, eat oysters!