Hot-Dip Galvanizing Vs. Weathering Steel

Hot-Dip Galvanized Steel vs.Weathering Steel

Hot-Dip Performance & Weathering Galvanized Steel Condition Steel Hot-dip galvanized steel is produced by Weathering steel is a corrosion-resistant dipping bare steel in a bath of molten zinc Unaffected by weathering Fatigue Life Reduced by weathering steel that initially corrodes.The presence of metal. A protective coating is formed by a corrosion products then limits any further metallurgical reaction between iron and Protects substrate steel Corrodes the same as oxidation of the metal. Constant Wetting zinc, providing both a barrier and cathodic from corrosion unprotected carbon steel protection that protets steel from corrosion. Provides excellent Corrodes the same as Faying Surfaces corrosion protection unprotected carbon steel

Prepare via sweep blast or Expensive preparation and Painting solvent cleaning excessive paint absorption Traps moisture inside, resulting in Protected inside and out Tubular Shapes increased corrosion rate Can’t distinguish patina rust from Visual and/or magnetic gauge Inspection loose corrosion products of accelerated corrosion Easily estimated for Corrosion Rate Unknown most environments

Good protection of substrate steel Road Salt Effect Accelerated corrosion/loss of mass

Appearance No visible corrosion products Stains concrete Problems HDG steel electrical distribution pole Weathering steel electrical distribution pole 90’ long x 6’ wide (approx.) Size Limitations None

Environmental Exposure

Poor corrosion protection, Good corrosion protection Sea Coast chlorides cause pitting

Chemical Poor corrosion protection, Good corrosion protection (Airborne) accelerated patina consumption

Moisture may accelerate corrosion, especially on faying surfaces, Excellent corrosion protection Vegetation enough to exert excessive force on bolted connections

Electrical Significant corrosion products No corrosion products Widely-used, no arcing Corrosion products cause arcing Industry

Excellent corrosion protection High-humidity/Fog Poor corrosion protection

American Galvanizers Association Phone: 720-554-0900 Fax: 720-554-0909 www.galvanizeit.org [email protected] Performance & Condition Notes Fatigue Life Tubular Shapes • Based on extensive studies of HDG steel in virtually • Weathering steel is progressively corroding, • Serious corrosion occurs on the inside of every atmospheric condition, HDG steel has over 80 sometimes at an increasing rate if moisture is tubular/enclosed shapes of weathering steel. years of history, and corrosion rates can easily be prevalent. Thus, there is a loss of steel mass and the Weathering steel will condense moisture inside, or estimated. More often than not, hot-dip galvanizing fatigue strength is lessened. This means that a design design characteristics will allow moisture to enter the protects steel beyond the expected life of a structure. using weathering steel may require thicker sections in enclosure and accelerate corrosion. There are light order to account for the loss of cross-section. poles that have failed, causing fatal consequences, and bridges exhibiting excessive corrosion. Road Salt Exposure • Hot-dip galvanized (HDG) steel protects the substrate • Weathering steel is adversely affected by exposure to steel and there is no loss of steel mass. • Regardless of moisture level, HDG steel provides road salts. Corrosion rates are accelerated, and the corrosion protection both inside and out. design strength is lessened. Constant Wetting • The chlorides in road salts make an aggressive attack • Weathering steel exposed to constant wetting will Inspection on the zinc coating of hot-dip galvanized steel, but corrode the same as unprotected carbon steel. • It is impossible to distinguish between the patina rust protection of the substrate steel from corrosion is Weathering steel needs to have a 50-50 wet/dry cycle that minimizes the corrosion rate of weathering steel still achievable. in order to form the stable patina of tightly-bound and the voluminous rust that occurs when the corrosion products. weathering steel is losing significant mass. Extensive and expensive testing is required to determine if Appearance • HDG steel will protect the underlying substrate for weathering steel has the design strength during the • Weathering steel will develop a dark brown, rather many years, regardless of the exposure to moisture. intended lifetime of the structure. consistent, patina over a three-to-five-year period. However, because the appearance often has an • HDG steel is inspected visually, and any substrate Painting architecturally aesthetic purpose, it is necessary to corrosion is obvious. Repairs can be made • Weathering steels are high-strength steels but, in blast or wire brush the surface in order to establish a immediately to extend the life of the steel. To general, are used because of their corrosion uniform initial appearance. When in contact with determine the expected life of the coating, a simple properties. Painting would seem to counteract the concrete, the corrosion products of weathering steel magnetic thickness measurement can be taken using logic of using weathering steels. However, weathering bleed onto the concrete and create an unsightly stain. inexpensive, hand-held tools. steel can be painted, but the patina is difficult to • HDG steel initially exhibits a bright silver-gray remove and the clean steel tends to require excessive appearance and then progresses within six to 24 paint material in order to deliver a decent Corrosion Rate months to the matte gray look that lasts for decades. coating appearance. • Weathering steel corrodes at unpredictable rates and The corrosion of zinc is slow, and the corrosion corrosion will accelerate when moisture is prevalent. • HDG steel does require preparation and, depending products are virtually invisible to the naked eye. Since corrosion is constantly occurring and the on the age of the coating, may range from a sweep weathering steel is losing cross-sectional thickness, blasting or solvent wipe to an economical hot- planning maintenance is unknown. water/steam pressure wash. Environmental Exposure Notes Sea Coast Exposure • Chemicals can be aggressive to HDG steel as well, Electrical Industry • Weathering steel performs poorly when exposed to but only after a number of years will the entire zinc • The buildup of weathering steel corrosion products salt air. Pitting and accelerated corrosion may coating be consumed. HDG steel is often painted in may allow arcing of electrical current from pole to compromise steel integrity. order to achieve a long-lasting coating to protect the pole. This may cause fires and power failures. • The zinc coating of HDG performs well in salt substrate steel. • HDG steel is used extensively by the power air exposure. industry for generation, transmission, and distribution, Vegetation without incident. Chemical Exposure (airborne) • The buildup of vegetation and organic material on and • Weathering steel does not perform well when around weathering steel concentrates moisture. High Humidity/Fog Exposure corrosive airborne chemicals are abundant. The Constant exposure to moisture is adverse to the • Weathering steel exhibits accelerated corrosion when patina is rapidly consumed and the steel corrodes at patina and excessive corrosion will occur. frequent high humidity and/or fog conditions exist. rates approximated to that of unprotected steel. • HDG steel is unaffected by the presence of vegetation • HDG steel is unaffected by such conditions as the or organic material. stable zinc patina is unreactive.