byFrank GoodwinWilliam MarquesRob White

International Zinc Association

The use of Galvanizing in the Mining sector in the World

• Mining projects require much steel-based infrastructure

• The environments can be severe (water and air) • Maintenance of structural components can be

difficult (limited and difficult access)• A robust corrosion protection system is required

Underground facilities

• Full water immersion• Long-term surface wetness, high humidity• Elevated temperatures• Acidity and alkalinity• Impact and abrasion

The mine environment can include:

Corrosion in South African mine shaft, uncoated, 20 years

service

Blasting effect of sharp stones on galvanized structure

Effect of pH on the rate of corrosion of zinc: (a)rapid corrosion(b) stable film – low corrosion rate(c) rapidcorrosion

Otherwise pure H2O !

Corrosion in Mine Waters

Mine water can vary in pH from 2.8 to 9.0, depending on origin and possible pretreatment

Mine water can be very hard, then it can contain salts that help create a stable protective film on Zn

In normal waters:

ZnO +HO Zn(OH) (both slightly insoluble)5Zn(OH) + 2CO Zn(OH)(CO) + 2HO (highly insoluble) 5Zn2+ + 2HCO

- + 8(OH) - Zn(OH)(CO) + 2HO (highly stable)

Fe + water = rust Fe + Zn + water Fe + Zn(OH) (cathodic protection and precipitation))

:

With Cl- contamination

With SO4-2 contamination

Other reactions are present

A galvanized coating will be attacked fairly rapidly at 1000 ppm Cl- levels in soft waterBut, similar levels of Cl- may be tolerated on galvanized surfaces in hard water (>100mg CaCO3).

Hard water, including water high in SO4-2, can precipitate

a deposit of protective Ca salt, such as carbonate or sulphate, on to the galvanized surface.

Such deposits retard the dissolution of underlying Zn as soluble ZnCl2 or ZnSO4

Rate of corrosion observed on a galvanized coating fully immersed for 100 daysin South African mine water having a pH of 5.7 (chloride 986 p.p.m., sulphate 270p.p.m., total dissolved solids2689 p.p.m.).

The mine environment

The mine environment

In high humidity, pH of condensed moisture on Zn is influenced by contamination from acidic gases (e.g. CO2 and NO2), also deposited dust contamination.

Example from Black Mt., S. Africa Pb/Zn mine, HDGASA inspection

Flowing water can cause erosion/corrosion

(Black Mt. HDGASA inspection)Use of cover to prevent flow

contact is effective

In many cases very satisfactory performance is seen

238 µm of Zn remaining 165 µm of Zn remaining

Hot dip galvanized shaft guides before installation

Surface Facilities

In many cases mine surface facilities are viewed as “chemical” environments (SO2, Cl-) where galvanized steel would not be applied, but many are benign as, apart from some acidic and alkaline reagents, generally in small quantities, their processing streams are at near to neutral pH values

Atmospheric Corrosion Corrosivity Categories

Life of galvanized coatings

Life of galvanized coatings to first maintenance

6500 tons of superstructure hot dip galvanized steel at Bafokeng- Rasimone Platinium Concentrator

Plant, S. Africa

Galvanized steel, Marula Platinum Smelter, S. Africa

Douglas Middelburg Opimisation (BHP Billiton)

Primary Crusher 26 km of Conveyors

Sishen Iron Ore Beneficiation Plant, S. Africa

Anglo Platinum’s Spud shaft

193µm of galvanized coating being indicated on horizontal hand railing.

101µm of galvanized coating being indicated on the “ball” of the stanchion.

Alcoa Pinjarra Alumina Refinery, Western Australia

Teck Red Dog, Alaska54 tons of galvanized

structurals

Actual costings

ESTIMATE OF TOTAL COSTS FOR STRUCTURAL STEEL PROJECT (ACTUAL PROJECT – 30 YEAR LIFE EXPECTANCY)

South African Rand (R)

Amount of Steel to be protected = 6300 tonnes Hot Dip Galvanizing 1 Heavy Duty Coating 2 Initial Cost of Application R9 072 000 R11 340 000 3 Total Initial Cost R9 072 000 R11 340 000 Future Maintenance Costs 4

Years 5 - 30 Nil R250 000 pa

Total Maintenance Costs Nil R8 250 000 Total Lifetime Cost R9 072 000 R19 590 000

Notes: 1. Actual Cost 2. Based upon – vinyl primer, vinyl intermediate, vinyl enamel finishing coat

to total DFT 200µm 3. Includes scaffolding, abrasive blast cleaning and all inspection 4. Based on experience at similar sites by consultant mining engineer and all

estimates at current prices including scaffolding and inspection

Environment Underground corrosive conditions

Surface plant corrosive conditions Surface plant moderate conditions

Primer coat (um)

Epoxy (40) Epoxy (100)

Epoxy (75)

Epoxy (75)

Epoxy (40)

Vinyl (15)

Intermediate coat (um)

Epoxy tar (90)

Vinyl (80)

Top coat (um) Epoxy tar (90)

Epoxy (75)

PU (50)

Epoxy (125)

HB vinyl (100)

Phenolic (100)

PU acrylic (30)

Vinyl (40)

ISO 12944-5 S9.11 S9.09 S9.01 S9.10

Typical duplex coating specifications for SA mining

industry

Remarks

• Mining industry applications can be severe, but many successful applications are realized

• Success depends upon:-Understanding the environment of application: use

test samples when situation uncertain-Understanding of how to design steelwork for

galvanizing-Work with fabricators on correct practices for

galvanizing-Good coordination between galvanizer and

contractors