Galvanizing Weathering Steel: When and Why
Weathering steel—including grades such as ASTM A588, A709 Weathering, and proprietary formulations like COR-TEN—can be successfully hot-dip galvanized despite these materials' inherent atmospheric corrosion resistance. While combining two corrosion protection systems might initially seem redundant, several practical scenarios drive the decision to galvanize weathering steel components.
The most common reasons include general availability and cost considerations at steel supply centers, situations where individual components for fabrication are only available in weathering steel grades, or cases where engineers specify ASTM A709 Grade 70W primarily for structural design requirements rather than corrosion protection strategy. Understanding the processing considerations and coating characteristics that result from galvanizing weathering steel helps specifiers, fabricators, and galvanizers achieve successful outcomes.
Material Availability and Specification Practices
Steel service centers and suppliers may stock weathering grades as standard inventory for certain structural shapes or thicknesses. When project schedules are tight or volumes are modest, procuring readily available weathering steel can prove more practical than ordering standard carbon steel grades with extended lead times. This availability-driven material selection occurs frequently in bridge construction, utility structures, and industrial applications.
Additionally, structural engineers sometimes specify ASTM A709 Grade 70W to meet strength requirements for member sizing without necessarily intending the weathering characteristics for corrosion protection. When hot-dip galvanizing is subsequently specified for corrosion protection, the weathering steel proceeds through the galvanizing process along with any standard carbon steel components in the same fabrication.
Steel Chemistry and Coating Development
Weathering steels achieve their atmospheric corrosion resistance through controlled alloying additions—primarily copper, chromium, nickel, and silicon. The silicon content, which can reach 0.40 percent in weathering grades compared to typical maximum values of 0.04 percent in standard structural steels, significantly influences galvanizing reactions and coating development.
Reactive Steel Characteristics
The elevated silicon levels found in weathering steels place them in the reactive steel category relative to hot-dip galvanizing. When silicon content falls within the reactive range—generally between 0.15 and 0.25 percent, or above 0.25 percent—the zinc-iron reaction during galvanizing proceeds more rapidly and extensively than with low-silicon steels. This accelerated reaction typically produces thicker, harder coatings with different visual appearance compared to standard galvanizing outcomes.
Weathering steel coatings commonly exhibit matte gray appearance with minimal or absent spangle pattern. The coating surface may feel rougher than typical galvanized finishes due to the predominance of zinc-iron alloy phases rather than the pure zinc outer layer found on low-reactivity steels. These characteristics result directly from the steel chemistry and represent normal outcomes for weathering steel galvanizing rather than process deficiencies.
Coating Thickness Considerations
The reactive nature of weathering steel frequently results in coating thicknesses exceeding typical values for standard structural steels. While ASTM A123 minimum thickness requirements apply regardless of steel grade, weathering steel coatings often substantially exceed these minimums. Coating thickness on weathering steel components may reach 6 to 10 mils or more compared to 3.9 to 5.5 mils typical for similar thickness carbon steel members.
Thicker coatings provide extended service life, which may be desirable for applications requiring maximum longevity. However, excessive coating thickness can occasionally create challenges with part fit-up, threaded connections, or coating durability during handling and installation. The harder, more brittle nature of thick alloy-layer-dominated coatings increases susceptibility to flaking if subjected to impact or severe bending during shipment or erection.
Specifiers should understand that weathering steel galvanizing inherently produces thicker, harder coatings than carbon steel. When dimensional tolerances are critical or connection details are sensitive to coating thickness variation, these characteristics require consideration during design and detailing.
Surface Preparation Requirements
Weathering steel surfaces present unique challenges for the chemical cleaning (pickling) processes that precede galvanizing. The protective oxide layers that form naturally on weathering steel—the same oxides that provide atmospheric corrosion resistance—resist removal by standard pickling procedures more tenaciously than mill scale on carbon steel.
Acid Pickling Considerations
Galvanizing facilities employing sulfuric acid for pickling often require extended cleaning times for weathering steel compared to carbon steel. Sulfuric acid's cleaning mechanism makes it particularly sensitive to the stable oxide layers on weathering steel, sometimes necessitating cleaning times two to three times longer than typical carbon steel processing.
Hydrochloric acid pickling proves somewhat more effective for weathering steel due to its more aggressive cleaning action and ability to penetrate beneath stable oxide layers. However, even with hydrochloric acid, weathering steel generally requires longer immersion times than carbon steel to achieve clean surfaces suitable for galvanizing.
Blast Cleaning Strategy
Many galvanizers and specifiers prefer abrasive blast cleaning for weathering steel prior to galvanizing. Blast cleaning provides several advantages: it removes all surface oxides and mill scale effectively regardless of steel grade, reduces or eliminates chemical pickling requirements, and can help moderate coating thickness development on reactive steels by slightly reducing the available iron surface for zinc reaction.
When blast cleaning is specified for weathering steel, the cleaning standard does not require specific surface profile or cleanliness degree parameters. However, Commercial Blast Cleaning per SSPC-SP 6/NACE No. 3 provides a reasonable reference specification. The objective is complete removal of mill scale and oxides rather than achieving particular surface roughness or anchor profile.
Blast cleaning adds cost compared to chemical cleaning alone, and not all galvanizing facilities maintain blast cleaning equipment. When blast cleaning is desired or necessary for weathering steel processing, early coordination with the galvanizer ensures capability alignment and appropriate cost estimation.
Material Test Report Communication
Providing Material Test Reports (MTRs) to the galvanizer before processing weathering steel enables proactive processing planning. The MTR silicon content and other alloy levels allow the galvanizer to assess reactivity, select appropriate surface preparation methods, and set appropriate expectations for coating characteristics. This advance communication helps avoid processing surprises and supports quality outcome planning.
Coating Performance and Protection
Despite differences in appearance and thickness, galvanized coatings on weathering steel provide corrosion protection equivalent to coatings on carbon steel when compared on a thickness basis. A given thickness of zinc coating—whether on weathering steel or carbon steel—delivers similar corrosion protection duration in any particular environment. The cathodic protection mechanism functions identically regardless of substrate steel grade.
Minor coating damage such as scratches or small bare spots benefits from the same sacrificial protection on weathering steel as on carbon steel. The zinc coating preferentially corrodes to protect exposed steel areas, and this protection operates independently of the substrate alloy composition.
Appearance Weathering
Initial appearance variations between weathering steel and carbon steel galvanized coatings diminish through natural weathering. While weathering steel components typically begin with matte gray appearance lacking spangle, and carbon steel may show bright metallic finish with visible spangle, both weather toward similar matte gray zinc patina appearance within months to a few years depending on atmospheric conditions.
For projects where initial appearance uniformity matters, specifiers should recognize that mixed-material fabrications (combining weathering and carbon steel) will likely show appearance differences initially. These differences moderate through weathering but may remain noticeable during early service life. When appearance consistency is critical, using uniform steel grades throughout the fabrication may be preferable.
Additional Weathering Steel Considerations
Weathering steels commonly contain copper as an alloying element, occasionally raising concerns about potential galvanizing kettle contamination. In practice, the copper levels in weathering steel (typically 0.20 to 0.40 percent) do not create kettle contamination concerns. Copper buildup in the zinc bath becomes problematic only when kettle copper concentration exceeds approximately 0.23 percent—a level not reached through normal processing of copper-bearing structural steels.
Pure copper cannot be galvanized because zinc requires iron for the metallurgical reactions that create the coating's alloy layers. However, copper-containing steels including weathering grades galvanize normally. The metallurgical reactions between zinc and iron proceed despite the presence of copper in the steel composition.
Design and Specification Guidance
When specifying hot-dip galvanizing for weathering steel components, several specification approaches help achieve successful outcomes. Identifying the steel grade and providing MTRs to the galvanizer enables appropriate process planning. If coating thickness control is critical for fit-up or connection performance, specify blast cleaning before galvanizing and consider including maximum coating thickness limits in addition to ASTM A123 minimums.
For projects where aesthetics matter, acknowledge that weathering steel may exhibit different initial appearance than carbon steel even when processed identically. If appearance uniformity is essential, consider specifying uniform steel grades throughout visible components or allowing additional weathering time before final inspection.
Communication among designer, fabricator, and galvanizer during project planning prevents misunderstandings about coating characteristics and establishes realistic expectations for appearance and thickness outcomes on weathering steel bridge components and other structures.
Frequently Asked Questions
Why galvanize weathering steel if it already resists atmospheric corrosion?
Material availability, structural design requirements, or project specifications drive the decision. Galvanizing weathering steel provides robust protection regardless of the base material's inherent characteristics.
Will galvanized weathering steel look different than galvanized carbon steel?
Yes, initially. Weathering steel typically produces matte gray coatings without spangle, while carbon steel may show bright, spangled appearance. Both weather toward similar gray patina over time.
Can weathering steel coating thickness be controlled?
Blast cleaning before galvanizing helps moderate thickness, but weathering steel inherently produces thicker coatings than carbon steel due to elevated silicon content. Design details should accommodate this characteristic.
Does weathering steel require special galvanizing procedures?
Extended pickling time or blast cleaning may be required, and coating inspection should recognize that thicker coatings are normal for weathering grades.
Expert Processing of Weathering Steel
V&S Galvanizing brings extensive experience in processing weathering steel grades including ASTM A588, A709 Weathering, and COR-TEN formulations. Our facilities are equipped for both chemical cleaning and blast cleaning, providing flexibility to optimize surface preparation for each project's requirements. We work collaboratively with fabricators and specifiers to establish appropriate processing approaches, set realistic coating expectations, and deliver quality outcomes on weathering steel projects of all types and sizes. Our technical team can review material test reports during project planning and recommend processing strategies that balance coating quality, project economics, and schedule requirements.
For additional technical guidance on weathering steel galvanizing, refer to the original AGA resource on galvanizing weathering steel.
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