BUILDING ENVELOPECORROSION

DESIGN ISSUESR.S. Charlton, P.Eng.

Levelton Engineering Ltd.

Prepared forBritish Columbia Building Envelope Council

June 14, 2000

Design Issues asRelated to Corrosion

Design Issuesl Choice of materialsl Type of coatingl Thickness of coatingsl Thickness of base materiall Wet/dry conditions

Due Diligence

l Have we done all that isreasonable in light of currentknowledge to prevent orminimize corrosionproblems?

Early Symptoms

l Early symptoms of possible corrosionor rot may not be evaluated andcorrected early enough, e.g.

u Musty odoursu Stainingu Wetnessu Condensation

Zinc Coating Life Expectancyl Impossible to predict preciselyl Depends on many factors

u Zinc coating thickness

u Sulphur pollution

u Proximity to sea coast

u Time and temperature at wetness

u Proximity to highways

u Agriculture and industry in area

Corrosion Rates ofZinc and Steel

Weight Loss (g)

Enviromment Steel Zinc

Steel/Zinc

Loss Ratio

Desert 2.2 0.13 17

Mild* 11.2 0.51 22

Semi-Industrial 23.8 0.84 28

Industrial 41.0 0.79 52

Marine 71.0 0.89 80

*Previously called rural

Source: Hot Dip Galvanizing - Lifetime Corrosion Protection, Doug Rourke, NACE CanadianRegion, Eastern Conference, Montrial, Quebec, November 14-16, 1989

Corrosion of Zinc in VariousAtmospheres

Enviromment

Years to Corrode

1 mil

Semi-arid 100+

Mild* 22

Southwestern Seacoast 14

Heavy Industrial Seacoast 4.6

Heavy Industrial 3.8

*Previously called rural

Source: Hot Dip Galvanizing - Lifetime Corrosion Protection, Doug Rourke, NACECanadian Region, Eastern Conference, Montrial, Quebec, November 14-16, 1989

Protection Offered by Zinc CoatingsCoating

Designation*Minimum Expected Service Life in Years to

Appearance of First Significant Rusting

Imperial Metric Mild Suburban Moderate Heavy

G-90 Z275 8.5 5.1 2.7 1.3

G-115 Z350 10.8 6.8 3.4 1.5

G-185 Z565 15.0 12.0 5.1 2.5

G-235 Z700 22.2 14.1 9.1 3.4

*Total both sides

Source: British Standard 5493, 1977, Table 3, Figure 1

Z275

G90

Ref: Galvanizing for Corrosion Protection - a Specifiers Guide, Figure 15,American Galvanizers Association, 1995

Treated Wood

l CCA (most common)l ACAl Boratel Others

Treated Wood - ACA/CCA

l Copper leached from woodl Precipitation reaction

u Steel

u Zinc

u Aluminum

l Galvanic corrosion

Galvanic SeriesCORRODED END

Anodic or less noble(ELECTRONEGATIVE)

ZincAluminumCadmium

Iron or SteelSoft Solders

CopperStainless Steels (passive)

Silver Solder

PROTECTED END

Cathodic or most noble(ELECTROPOSITIVE)

Anode - the electrode at whichoxidation or corrosion occurs

(opposite of cathode)

Zn →→ Zn+2 + 2 electrons

Cathode – the electrode wherereduction and normally nocorrosion occurs (opposite ofanode)

4 electrons + O2 + 2 H2O →→ 4 OH-

Galvanic Couple

Zn, Fe or AlBase metal - anode

Cu deposited onsurface - cathode

Rapid corrosion

Zinc Coatingsl Hot-Dip Galvanizing (HDG)l Continuous Galvanizingl Mechanical Galvanizingl Electrogalvanizingl Zinc Platingl Metallizingl Sherardizingl Zinc Painting

Hot-Dip Galvanizingl Bath of molten zinc metall Metallurgical reactionl Zinc-iron alloy layersl Wide size range possiblel Thickness normally greater than

specificationl 3.4 mil (86 µµ) coating typicall 6-8 mils (150-200 µµ) common on structural

steels

Zinc Coating Thicknesses

l 100 gm/m2 equals 14.3 µµl 2 oz/ft2 equals 3.4 mils

Ref: Hot Dip Galvanizing Costs Less and Lasts Longer, Figure 2, American GalvanizersAssociation, 1997

Ref: Galvanizing for Corrosion Protection - a Specifiers Guide - Figure 9,American Galvanizers Association, 1995

Continuous Galvanizingl Hot-dip galvanizing processl Used for coating steel sheet, strip or

wirel Coating almost pure zincl Standard product G-90/Z275l Zinc-aluminum alloys

u 55% Al - 45% Znu 95% Zn - 5% Al

Hot-Dip Coated Steel SheetCoating Designations*ASTM

Specification TypeInch Pound Units S1 Units

A 653 Zinc G30-G360 Z120-Z1100

Zn-Fe A25-A60 ZF75-ZF180

A 792 Al-Zn AZ50-AZ60 AZ150-AZ180

A 875 Zn-Al GF30-GF235 ZGF90-ZFG700

*Total weight both sides

Mechanical Galvanizingl Used for coating small parts by drum tumbling

u Propriety promoter chemicalsu Zinc powderu Glass beads

l Thicknesses range from 0.2-4.3 mils (5-110 µµ)

l Commercial fasteners 2 mils (50 µµ)l Coating > 30% less dense than HDG

l Alternate method to HDG in ASTM Fastener

Specifications

Ref: Zinc Coatings - American Hot Dip Galvanizers Association, Inc.MA-14, 1988

Electrogalvanizing

l Used for zinc coatings onsteel sheet and strip

l Coating weights both sidesup to 0.2 oz/ft2 (60 g/m2)

l Thicknesses of up to 0.14 mil(3.6 µµ) per side

Zinc Platingl Electrodeposition processl Used for small parts such as

fastenersl Thin coating < 1 mil (25 µµ)l Only suitable for very mild

(indoor) exposures

Metallizing

l Melted zinc is sprayed on surface ofpart to be coated

l White metal abrasive blast requiredl Usually sealed with an organic

coatingl Coatings in excess of 10 mils (250 µµ)

Sherardizingl Not widely used in North Americal Used for fastenersl Parts charged with powdered zinc

and fluxl Tumbled at temperature less than

melting point of zincl Coating formed by diffusionl Maximum thickness ≈≈ 1 mil (25 µµ)

Zinc Paintingl Paints contain 92-95% metallic

zinc in dry filml Classified as organic or inorganicl White metal or near white blasting

requiredl Normal thickness: 2.5-3.5 mils

(64-90 µµ)

Ref: American Galvanizers Association

Steel Sheet Hot-Dip ProcessSpecifications

l ASTM A 653/A 653M “Standard Specification for SteelSheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process”

l ASTM A 924/A 924M “Standard Specification forGeneral Requirements for Steel Sheet, Metallic-Coatedby the Hot-Dip Process”

l ASTM A 792/A 792M-94 “Standard Specification forSteel Sheet, 55% Aluminum-Zinc Alloy-Coated by theHot-Dip Process”

l ASTM A 875/A 875M-94 “Standard Specification forSteel Sheet, Zinc-5 % Aluminum Alloy-Coated by theHot-Dip Process”

Zinc Coating Specifications

l ASTM B 695-91 “Standard Specification for Coatingsof Zinc Mechanically Deposited on Iron and Steel”

l ASTM A 591/A 591M-98 “Standard Specification forSteel Sheet, Electrolytic Zinc-Coated, for Light CoatingMass Applications”

l ASTM B 633-85 “Standard Specification forElectrodeposited Coatings of Zinc on Iron and Steel”

l ASTM A 123-89a “Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products”

l CAN/CSA-G164-M92 “Hot Dip Galvanizing ofIrregularly Shaped Articles”

Adhesion of MetallicCoatings

l ASTM B 571 “Standard TestMethods for Adhesion ofMetallic Coatings”

u 3.1 Bend Testsl Mandrel diameter 4 X t

Building Envelope Design

l Has the building envelopebeen designed to prevent orminimize corrosion problemsand provide a long andproblem-free life ?