20070822chapterf_technote

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Durability of Cold-Formed Steel Framing Members
Summary: The purpose of this document, its third edition, is to give engineers, architects, builders and home and
commercial building owners a better understanding of how galvanizing (zinc and zinc alloy coatings) provides long-term
corrosion protection to cold-formed steel framing members. This document also suggests guidelines for selecting, handling
and using these steels in framing applications.
1.0 INTRODUCTION
minute (200 meters/minute). As the steel exits the moltenzinc bath, air “knives” blow off the excess coating from the The purpose of this document is to give engineers, steel sheet and control the coating thickness to the architects, builders and home and commercial building specification requirement. The coated sheet steel is owners a better understanding of how galvanizing (zinc chemically treated (passivated) to protect against storage and zinc alloy coatings) provides long-term corrosion stain and, if requested, oiled and recoiled for shipment to protection to cold-formed steel framing members. This document also suggests guidelines for selecting, handlingand using these steels in framing applications.
Extensive information on the properties, performance andapplications for zinc and zinc alloy coated steel sheet can 1.1 Design Life
be found on the GalvInfo Center web site atwww.galvinfo.com.
A home is one of the few necessities that consumersexpect to last a lifetime or more. For commercial propertyowners, each structure represents a significant invest- 2.2 Types of Coatings
ment. It is critical therefore, that the framing materialperforms its function for as long as other critical compo- The continuous galvanizing process can apply a number nents, such as the roof structure, exterior and interior wall of different coatings that vary in thickness, appearance coverings, and flooring. For sufficient longevity, cold- and alloy composition. A number of coating compositions formed steel framing needs proper corrosion protection.
are acceptable for cold-formed steel framing, as noted inASTM A1003/A1003M2. The common North American Galvanizing is one of the most economical and effective coatings, according to AISI S2013, are as follows: ways to protect steel. Steel framing materials used inresidential and light commercial construction can be • Zinc-Coated (Galvanized): The name, galvanized, usually effectively protected by a galvanized coating.
refers to the “standard” continuous coating that isbasically pure zinc. About 0.2% aluminum is added to the 2.0 GALVANIZING
galvanizing bath to form a thin, inhibiting, iron-aluminumlayer on the steel surface that ensures formation of a pure 2.1 Definition
zinc coating. The finished coating has good formability andcorrosion resistance, and provides excellent sacrificial Galvanizing is a process whereby steel is immersed into a protection (Section 4.2). For further details on coating bath of molten zinc (865°F/460°C) to form a metallurgically specifications, refer to ASTM A653/A 653M4. bonded zinc coating. This same hot dip immersion processis also used to produce aluminum-zinc alloy coatings.
• 55% Aluminum-Zinc Alloy-Coated: This is a 55% Most cold-formed steel is galvanized by unwinding coils aluminum, 43.5% zinc, and 1.5% silicon alloy coating that of cold rolled steel and feeding the sheet continuously provides superior barrier corrosion resistance over through a molten zinc bath at speeds up to 600 feet per galvanized coatings. For further details on coatingspecifications, refer to ASTM A792/A792M5.
Cold-Formed Steel Engineers Institute September 2007
Table 2.1
Zinc Coating Weights (Mass) / Thickness
Minimum Requirement
Thickness
Total Both Sides
Nominal per Side
Coating Designation
(oz/ft2)
(microns)
Zinc (Galvanized)
55% Aluminum-Zinc
2.3 Types of Surface Finishes
scribed by AISI S2013. For further details on coatingspecifications, refer to ASTM A653/A653M4 (zinc- Zinc and zinc alloy coatings can differ in appearance coated) and A792/A792M5 (55% aluminum-zinc based on the size of spangle or type of surface finish.
coated). Coating requirements for framing members Spangle is the flowery pattern that results as molten zinc also appear in ASTM C6456 and C9557.
grains grow and are then frozen in place as the coatingsolidifies. Spangle size can be controlled or eliminated by A heavier coating may be advisable in applications various processing techniques. Spangle control does not where the environment is particularly corrosive.
effect coating thickness so the presence or absence of Section 3 contains more information on the spangle has no influence on corrosion performance performance of zinc coatings in various environ- ments and identifies areas where additionalprotection may be required.
2.4 Coating Weights and Thicknesses
The galvanizer controls how much coating is put on the 3.0 DURABILITY OF GALVANIZED STEEL
steel. The amount of coating is measured by coating weight (ounces per sq. foot, grams per sq. meter) or bythickness (mils, microns). Table 2.1 lists the common The durability of zinc-based coatings is a function of time commercially available continuously galvanized coatings of wetness and composition of the atmosphere (refer to used for cold-formed steel framing members.
Section 4 for details). Since residential galvanized steelframing is intended for dry indoor environments, the Table 2.2 lists the minimum coating requirements for corrosion rate of zinc should be very low. According to the structural and non-structural framing members, as pre- corrosion rates in Section 3.1 and the minimum coating Table 2.2
Minimum Coating Weight Requirements
Framing Member
Zinc (Galvanized)
55% Aluminum-Zinc
Designation
September 2007
Cold-Formed Steel Engineers Institute thicknesses specified in Table 2.2, zinc based coatings can Additionally, a survey was performed in May 1995 on a 20- protect steel for the design life of the structure.
year old steel-framed house in Stoney Creek, Ontario11. Theinspection revealed no visible signs of corrosion of the Just as water leakage, excessive humidity or condensation zinc coating or the steel studs. Coating thickness measure- will damage most construction materials over time, so will ments taken on exterior and interior studs showed no it accelerate the corrosion of zinc coatings. However, if a measurable loss in coating thickness.
building is built to code and properly ventilated andmaintained, moisture should not be a concern for galva- 3.2 Interior Walls
Interior, non-load bearing walls will likely experience the Additional corrosion protection is recommended for most benign atmosphere in the structure. It is unlikely that structures built in particularly aggressive environments; these steel members will be subject to moisture on a i.e., humid coastal areas (CFSEI document8).
regular basis and the coatings specified in Table 2.2 shouldgive adequate protection.
3.1 Performance in Service
Venting of rooms that generate considerable amounts of The corrosion rate of zinc coatings in an indoor atmo- moisture (i.e. bathroom, kitchen) should be to the outside, sphere of a building’s structure is generally low. Accord- ing to a three-year British Steel study9, the corrosion ofzinc is lower than 0.1 µm (microns) per 3-year period in 3.3 Exterior Walls
houses located in different rural, urban, marine andindustrial atmospheres (Figure 3.1). This indicates that Proper design and building practices which include vapor under similar conditions a 10-µm (microns) zinc coating retarders, thermal breaks, and air barriers should eliminate should last for more than 300 years.
significant moisture transfer into exterior wall cavities, andthus prevent the framing from being exposed to significant A recent seven-year study conducted by the NAHB moisture accumulation. Galvanized cold-formed steel framing Research Center10 measured coating corrosion rates of in accordance with Table 2.2, should adequately protect the test samples installed in the exterior wall cavities and framing when enclosed. Framing that is intentionally left ventilated crawl and attic spaces of four houses located in exposed (e.g., supports for a balcony penetrating a wall, a different geographic locations (inland, marine and garage with exposed framing, etc.) may require additional industrial) in North America. Based on the corrosion rates protection as the exterior environment is considered to have measured, the estimated life expectancy of the coating access to the framing. Heavier coatings will also be required varies from 325 years to over 1000 years.
for aggressive industrial and coastal environments8 .
Corrosion loss of galvanized steel, exposed in the loft of 15 residential houses located in three differentgeographical areas in England, UK, as a function of exposure time (John, 1991). The data points are the meanvalues of 6 samples for each house; the equation in the figure is the best fit from linear regression analysis.
Figure 3.1
Cold-Formed Steel Engineers Institute September 2007
Particular attention should be paid to the bottom track of 4.1 Barrier Protection
exterior walls that may collect moisture during installationor during the service life of the wall. A vapor barrier or sill The main mechanism by which galvanized coatings gasket can be installed between the track and the founda- protect steel is by providing an impervious barrier that tion to prevent underside corrosion in the event that the does not allow moisture to contact the steel. Without moisture (the electrolyte) there is no corrosion. The natureof the galvanizing process ensures that the metallic zinc 3.4 Floor Framing
coating has excellent coating adhesion and abrasionresistance.
Floor joists contained within the controlled environmentof a building or structure are not likely to be exposed to Galvanized coatings will not mechanically degrade (crack, aggressive conditions, as a result the corrosion coating de-bond or fade) over time as will other barrier coatings values specified in Table 2.2 should adequately protect the such as paint. However, zinc is a reactive material and will framing. Framing that is exposed to environments open to corrode slowly over time (Figure 3.1). For this reason, the the outside, such as vented basements or crawlspaces, protection offered by galvanized coatings is proportional unconditioned basements, and the like should have greater protection. Floor joists installed in a basement orcrawlspace not having a concrete slab floor or vapor 4.2 Cathodic Protection
barrier at the floor should be considered the same ashaving the framing exposed to the exterior environment.
Another important protection mechanism is zinc's ability Greater corrosion protection is recommended in these to galvanically protect steel. When base steel is exposed, applications due to the likelihood of exposure to extended such as at a cut or scratch, the steel is cathodically periods of high humidity. Floor joists or tracks that are protected by the sacrificial corrosion of the zinc coating attached directly to the foundations made of concrete or adjacent to the steel. This occurs because zinc is more pressure treated wood can include a vapor barrier or sill electronegative (more reactive) than steel in the galvanic gasket under the joist or track to prevent underside corrosion in the event that the foundation gets wet. Forall of the above scenarios, proper building design and In practice, this means that when underlying steel is building practices which include the installation of exposed by coating damage or at a cut edge, the zinc insulation, vapor retarders, thermal breaks, and air barriers coating will not be undercut by rusting steel (Figure 4.1) should eliminate significant moisture transfer and accumu- because the steel cannot corrode adjacent to the zinc Table 4.1
3.5 Roof Framing
Galvanic Series of Metals
and Alloys (in seawater)
Attics normally contain some form of cross ventilation due Corroded End - Anodic
to the provisions of the locally adopted building code.
Roof framing is generally considered to have more exposure to the exterior environment than other areas of the building’s structure and increased corrosion protec- tion may be advisable for buildings located near salt water or industrial environments where the atmosphere is more corrosive8. Also, a prolonged roof leak may cause localized corrosion of the affected roof members.
4.0 CORROSION PROPERTIES OF ZINC
It is well known that steel rusts when left unprotected in Protected End - Cathodic or most noble
almost any environment. Applying a thin coating of zinc to steel is an effective and economical way to protect steelfrom corrosion. Zinc coatings protect steel by providing a Note: Any one of these metals and alloys will physical barrier as well as cathodic protection to the theoretically corrode while protecting any other that is lower in the series as long as both form part of an September 2007
Cold-Formed Steel Engineers Institute FIGURE 4.1 (A): ZINC COATING
FIGURE 4.1 (B): PAINT COATING
4.3 Corrosion Process
The pH of the atmosphere, rain or other liquids that The ability of a zinc coating to protect steel depends on contact zinc have a significant effect on corrosion rate.
zinc’s corrosion rate. It is therefore important to under- Moderately acidic conditions or fairly strong basic stand zinc’s corrosion mechanism and what factors affect conditions may increase zinc's corrosion rate. Most industrial atmospheres contain sulfur in the form of sulfurdioxide and sulfuric acid, which are corrosive to zinc.
Freshly exposed galvanized steel reacts with the surround-ing atmosphere to form a series of zinc corrosion products.
Chloride environments (i.e. marine) have a lesser effect on In air, newly exposed zinc reacts with oxygen to form a thin zinc’s corrosion rate than sulfur compounds. Neverthe- zinc oxide layer. When moisture is present, zinc reacts with less, because chlorides can be prevalent in coastal water resulting in the formation of zinc hydroxide. A final environments, chlorides may likely be of concern necessi- common corrosion product to form in atmosphere is zinc tating extra corrosion protection8.
carbonate as zinc hydroxide reacts with carbon dioxide inthe air. Other zinc compounds containing sulfate or 4.4 Wet Storage Stain
chloride can also be present in the corrosion productsformed in industrial or marine environments. The zinc “Wet Storage Stain” is a term traditionally used in the corrosion products formed in atmospheric environments galvanizing industry to describe the white zinc corro- are usually a thin, tenacious and stable layer that sion product that sometimes forms on the galvanized provides protection to the underlying zinc. These steel surfaces during storage and transport.
corrosion products are what give zinc its low corrosionrate in most environments.
When freshly galvanized steel is stored or installed withmoisture trapped behind contacting surfaces and access Zinc corrosion rates correlate with two major factors; to free-flowing air is restricted, zinc hydroxide may form.
time of wetness, and concentration of air pollutants12,13.
This is a voluminous, white, non-protective corrosion Corrosion only occurs when the surface is wet. The product. Zinc hydroxide can form during a single incident effect of wetting on zinc’s corrosion rate depends on of wetting, by rain or condensation; however, once the the type of moisture. For example, while the moisture affected areas are exposed and allowed to dry, it generally from rainfall may wash away zinc’s corrosion products has little harmful effect on the long-term performance of causing further zinc corrosion to occur, moisture formed galvanized steel. If the damp, restrictive conditions by condensation usually can evaporate and leave the continue, then zinc corrosion may proceed rapidly down corrosion products in place. Since steel framing inside to the base steel. Most galvanized sheet products receive buildings should be dry almost all of the time, zinc’s some form of surface treatment (passivation) to help prevent the formation of wet storage stain.
Cold-Formed Steel Engineers Institute September 2007
4.5 Contact with Non-Metallic Materials:
other. The reaction will only occur when the dissimilarmetals are connected to form an electrical circuit and an • Mortar and Plaster: Damp freshly prepared mortar and electrolyte (such as moisture) is present. It is this plaster may attack zinc and zinc alloy coatings, but reaction that is responsible for the galvanic protection corrosion ceases when the materials become dry. Since of steel by zinc coatings at the place where the coating these materials absorb moisture, care should be taken to either keep them dry or isolate the steel framing fromthe plaster or mortar.
Based on outdoor atmospheric studies12,13. Table 4.2 • Wood: Galvanized steel does not react with dry wood.
presents the relative galvanic corrosion rate of zinc Galvanized steel can be safely fastened to wood that is when coupled to various metals. In normal indoor dry throughout most of its service life.
environments moisture levels are low and consequently the galvanic action between dissimilar metals is much Pressure Treated Wood: There are a variety of lower than in outdoor environments. The galvanic chemicals used to pressure-treat wood to help protect interaction between dissimilar metals is complex and the components from attack by termites, other insects, expert opinion should be sought on the advisability of and fungal decay. Recent testing has indicated copper- combining different materials. The extent of the galvanic based products, i.e. ACQ, CA and ACZA are more action depends on the metals coupled. Advice given corrosive to galvanized steel than the former CCA, should fall into one of three categories: which was voluntarily withdrawn from the market for 1) The choice is satisfactory and unlikely to cause a many applications in 2003. Viable options for cold- formed steel framing that should be considered would seem to include specifying the less corrosive sodium 3) Use the materials selected but electrically insulate the borate (SBX/DOT) pressure treatment, isolating the steel and wood components, or avoiding use of pres-sure treated wood14.
According to Table 4.2, galvanic corrosion of zinc is the • Gypsum Sheathing (Drywall) and Insulation ducts: most severe when in contact with steel, copper or brass Dry wall and various dry insulating products (mineral under moist conditions. If contact between galvanized wool, cellulose and rigid foam) do not react with coatings and copper/brass or bare steel cannot be avoided, then insulated, non-conductive gaskets • Concrete: Freshly poured concrete may react with should be used at the contact points to prevent galvanized coatings because it is wet and highly localized consumption of the galvanized coating.
alkaline (pH 12 to 13). However, as the concrete curesand dries, it becomes non-aggressive to these coatings.
On the other hand, contact between galvanized coat- Since curing times are relatively short, the corrosion of ings and aluminum or stainless steel results in less bi- the coating is minimal. Good quality concrete that is metallic corrosion. However, insulating the materials free of chlorides is not corrosive to zinc.
may be advisable in humid environments.
4.6 Contact with Other Metals
Bi-metallic interaction is an electrochemical reactionthat can occur between some dissimilar metals or alloysthat causes corrosion of one metal and protection of the Coupled Alloy
Zinc Corrosion Rate
Table 4.2
Galvanic Corrosion Rate of Zinc Coupled to Other Common Commercial Metals12,13
September 2007
Cold-Formed Steel Engineers Institute 5.0 BUILDING WITH GALVANIZED STEEL
5.4 Fasteners
5.1 Fabricating Galvanized Framing Members
Steel framing fasteners are usually protected againstcorrosion by electroplated zinc coating. Zinc plated Galvanized steel is shipped to fabricators as coils.
coatings are typically thinner than and therefore not as The coils are slit lengthwise into individual “rib- protective as the galvanized coatings on the surround- bons” of galvanized steel strip. These ribbons are roll-formed, cut to length and holes are oftenpunched to produce the various steel sections used For more aggressive environments, improved fastener corrosion protection can be achieved with differentorganic, plated or even duplex (i.e. two or more com- Galvanized zinc coatings are metallurgically bonded bined coating systems) coatings. Fastener suppliers to the steel sheet and will not spall or flake off can provide further information on the level of corro- during these forming operations. Zinc also cathodi- sion protection that is recommended for particular cally protects any steel exposed at cut edges (Sec- environments and the level of protection provided by 5.2 Storage, Handling and Installation
6.0 CONCLUSIONS
A galvanized coating is adherent and abrasion resistant.
As a result, normal handling during distribution, Zinc and zinc alloy hot dip galvanized coatings are storage and installation should not damage the zinc economical and recommended methods of providing coating. Coating damage that results from necessary long-term corrosion protection of steel framing mem- job site procedures such as shearing, cutting or fastening is mitigated by zinc’s ability to cathodicallyprotect any exposed steel at cut edges.
The galvanizing process produces a tough metalliccoating that can withstand the physical demands Precautions should be taken to avoid the formation of created during fabrication, distribution, site storage and wet storage stain (Section 4.4) at all points in the installation of the steel-framing members.
distribution cycle and when storing galvanized steel atthe job site. The galvanized steel should be stored to Time of wetness and concentration of air pollutants allow proper drainage and good ventilation so that all affect zinc’s corrosion rate and situations that expose surfaces can dry after becoming wet.
steel framing to extended periods of wetness or aggres-sive atmospheres should be avoided. Thicker zinc In particularly aggressive conditions such as humid coatings or additional topcoats can be specified for coastal environments, extra effort should be taken to increased corrosion protection in areas where aggres- minimize outdoor exposure of the galvanized framing members during storage and installation8.
However, in most indoor or sheltered environments 5.3 Welding
where steel framing is used (i.e. enclosed walls andfloors, framing members that are not directly exposed to Galvanized steel can be joined by spot or continuous moisture and aggressive atmospheres (i.e. salty marine welding. Welding may be an economical joining method air etc.)) the corrosion rate of zinc and zinc alloy when shop fabricating wall or roof assemblies. Al- coatings is very low. With the use of recommended though both welding operations volatilize the zinc coating weights, as described AISI S2013, steel framing coating at the weld site, spot welding is a much more members will function for hundreds of years.
Spot welds and continuous welds will remove thezinc coating. Damaged areas should be repairedusing zinc rich touch-up paint or by zinc metallizingin accordance with ASTM A78015. Zinc metallizing isa thermal process that propels molten zinc particlesonto the steel substrate producing a continuousmetallic zinc coating.
Cold-Formed Steel Engineers Institute September 2007
CONTRIBUTORS
This publication was developed by the Corrosion and Durability Task Group, a joint effort of the Committee on Framing Standards of the American Iron and Steel Institute and the Cold-Formed Steel Engineers Institute, a Council of the Steel Framing Alliance. This publication is intended to provide designers with guidance in selecting coated steels and enhancing durability in buildings that utilize AISI and SFA acknowledge Douglas J. Rourke of the International Zinc Association and X. Gregory Zhang of Teck Cominco Metals Ltd. as the primary authors of the first (1996) edition of this document, and are grateful to thefollowing members of the Corrosion and Durability Task AISI and SFA also wish to express their appreciation for Group who helped develop this third edition.
the support of the American Zinc Association.
References
1. Townsend, H.E., “Continuous Hot Dip Coatings”, Metals Handbook, ASM International, Materials Park, OH, 1995.
2. ASTM A1003/A1003M, Standard Specification for Steel Sheet, Carbon, Metallic- and Nonmetallic-Coated for Cold-FormedFraming Members, - ASTM International, West Conshohocken, PA, www.astm.org.
3. North American Standard for Cold-Formed Steel Framing - Product Data, AISI S201, American Iron and Steel Institute, Washing-ton, DC, 2007.
4. ASTM A653/A653M, Standard Specification for Sheet Steel, Zinc Coated (Galvanized) or Zinc-Iron Alloy Coated (Galvannealed)by the Hot Dip Process, - ASTM International, West Conshohocken, PA, www.astm.org.
5. ASTM A792/A792M, Standard Specification for Steel Sheet, 55% Aluminum-Zinc Alloy Coated by the Hot Dip Process, - ASTMInternational, West Conshohocken, PA, www.astm.org.
6. ASTM C645, Standard Specification for Nonstructural Steel Framing Members, ASTM International, West Conshohocken, PA,www.astm.org.
7. ASTM C955, Standard Specification for Load-Bearing (Transverse and Axial) Steel Studs, Runners (Tracks), and Bracing orBridging for Screw Application of Gypsum Board and Metal Plaster Bases, ASTM International, West Conshohocken, PA,www.astm.org.
8. Corrosion Protection for Cold-Formed Steel Framing in Coastal Areas, Technical Note D200, Cold-Formed Steel EngineersInstitute, Washington, DC, 2007.
9. John, V., Durability of Galvanized Steel Building Components in Domestic Housing, British Steel Technical - Welsh Laboratories,Port Talbot, UK, 1991.
10. ILZRO, ZC-4, Seven Year Report, Galvanized Steel Framing for Residential Buildings, International Lead Zinc ResearchOrganization, Research Triangle Park, NC, 2006.
11. DeMeo, L.D., “20 Year Inspection of LSF at DESH”, unpublished report, Dofasco Inc., Hamilton, Ontario, Canada, 1995.
12. Zhang, X.G., Corrosion and Electrochemistry of Zinc, Plenum Publishing Corporation, New York, NY, 1996.
13. Zhang, X.G., “Corrosion of Zinc and Its Alloys”, Metals Handbook, ASM International, Materials Park, OH, 2005.
14. Pressure Treated Wood and Steel Framing, Issue Paper 4, Steel Framing Alliance, Washington, DC, 2007.
15. ASTM A780, Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings, ASTM Interna-tional, West Conshohocken, PA., www.astm.org.
16. Corrosion Protection of Screw Fasteners, Technical Note D100, Cold-Formed Steel Engineers Institute, Washington, DC, 2007.
This “Technical Note on Cold-Formed Steel Construction” is published by the Cold-Formed Steel Engineers Institute (“CFSEI”). The
information provided in this publication shall not constitute any representation or warranty, express or implied, on the part of CFSEI
or any individual that the information is suitable for any general or specific purpose, and should not be used without consulting with a
qualified engineer, architect, or building designer. ANY INDIVIDUAL OR ENTITY MAKING USE OF THE INFORMATION
PROVIDED IN THIS PUBLICATION ASSUMES ALL RISKS AND LIABILITIES ARISING OR RESULTING FROM SUCH
USE. CFSEI believes that the information contained within this publication is in conformance with prevailing engineering standards of
practice. However, none of the information provided in this publication is intended to represent any official position of the CFSEI or
to exclude the use and implementation of any other design or construction technique.
September 2007
Cold-Formed Steel Engineers Institute

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