November 12, 2019
By Alana Hochstein
As specifiers of commercial buildings look to achieve durable and long-lasting structures, many are turning to hot-dip galvanizing (HDG) as a method for protecting structural-steel members against atmospheric corrosion for many decades. However, when HDG is specified as the primary method of corrosion protection for structural steel members requiring a fire-resistance rating, passive fireproofing specifications may need to be lengthy and detailed to ensure the overall system will perform as the manufacturer intended. Although there are many benefits to using HDG steel for corrosion protection under fireproofing, the galvanized coating must be carefully prepared for the system to meet requirements for minimum bond strength and perform as intended.
The term galvanizing is often used to describe a variety of coating methods used to apply zinc to steel for the purpose of corrosion protection. Each of the following application methods have unique characteristics affecting the type of products that can be coated, coating thickness, economics, and performance in the environment:
Of these, batch HDG, also known as general galvanizing, is the most common method used for structural steel members in building construction. Therefore, this article reviews specific recommendations for the application of passive fireproofing over batch HDG steel.
Batch HDG involves submerging iron or steel products into a bath of molten zinc to produce a thick, metallurgically bonded coating comprising zinc/iron intermetallic alloy layers and a surface coat of pure zinc (Figure 1). This process is performed after the steel member’s fabrication such as poles, beams, plates, frames, and other assemblies. First, the steel products are chemically cleaned to remove organic contaminants, mill scale, and oxides prior to immersion in a zinc bath (galvanizing kettle) containing at least 98 per cent pure zinc and heated to approximately 443 C (830 F).
While the steel is immersed in the galvanizing kettle, zinc reacts with iron to form the hot-dip galvanized coating, becoming part of the steel itself rather than just a surface/barrier coating. This metallurgical bond creates a barrier coating with exceptional resistance to abrasion because the intermetallic layers of the coating are harder than the substrate steel (read the 2011 publication “Zinc Coatings” by the American Galvanizers Association [AGA]). The HDG coating also provides the steel with cathodic protection ensuring the steel underneath will not corrode until all nearby zinc has been consumed. When small surface damage occurs exposing bare steel, the surrounding zinc will still provide cathodic protection, unlike painted steel.
Another aspect of HDG differentiating it from other building materials is its longevity in atmospheric environments. The time to first maintenance of a hot-dip galvanized coating depends on the coating thickness and the corrosivity of the environment, but it is not uncommon to experience a longevity of 50 to 120 years for batch HDG items depending on atmospheric conditions such as yearly average rainfall, temperature, humidity, air salinity, and exposure to road salts (Figure 2) (see the 2014 publication “The Performance of Hot-dip Galvanized Steel Products in the Atmosphere, Soil, Water, Concrete and More” by AGA).
HDG for corrosion protection under passive fireproofing
Although HDG has long been recognized for its ability to provide protection against atmospheric corrosion in aggressive environments, there are additional benefits when employing spray-applied fire-resistive materials (SFRMs) and intumescent fire-resistive materials (IFRMs) atop galvanized surfaces (see the 2014 publication “The Performance of Hot-dip Galvanized Steel Products in the Atmosphere, Soil, Water, Concrete and More” by AGA).
Some SFRMs, such as cementitious coatings, are unable to exclusively provide significant corrosion resistance in aggressive environments, or can be subject to corrosion under fireproofing when damaged or on retention of moisture in the pores of the SFRM. The resulting products of steel corrosion (e.g. iron oxides) are more voluminous than the base steel (rusting steel expands), and the ensuing pressure can lead to delamination of SFRMs and failure to protect structural members during a fire (consult the white paper “Corrosion Under Insulation” by the Materials Technology Institute [MTI], published in 2012). Meanwhile, field studies conducted on the corrosion performance of hot-dip galvanized steel under cementitious passive-fireproofing materials have demonstrated the ability to provide over 48 years of maintenance-free longevity in corrosive interior environments containing high levels of humidity and/or chemical exposure (refer to “Corrosion of Galvanized Steel Under Fireproofing” by A.M. Stoneman and F.E. Goodwin). The HDG provides a barrier and cathodic protection to prevent the buildup of iron oxides, with the addition of superior abrasion resistance. However, site conditions that could result in damage to fireproofing that exposes the coating to chemical environments or solutions with pH values below three or above 13.5 are not recommended due to the rapid corrosion of the galvanized coating (refer to “Corrosion of Galvanized Steel Under Fireproofing” by A.M. Stoneman and F.E. Goodwin). In other aggressive environments, proper inspection and maintenance of the fireproofing material and replacement of any damaged material is important to prevent accelerated corrosion of the galvanized steel.
A subset of SFRMs, IFRMs purposefully char and swell dramatically on exposure to fire, and the charred layers protect the steel from a fire. Intumescent coating systems provide corrosion resistance in addition to fire-resistance since acrylic, vinyl, or epoxy primers are often specified for improved bond-strength and barrier protection. However, when IFRMs are applied over a galvanized surface using a compatible tie-coat recommended by the IFRM manufacturer, a more sophisticated corrosion resistance, known as the synergistic effect, is achieved. The IFRM coating system prevents the galvanized steel from exposure to the environment, thereby delaying the onset of zinc corrosion. In return, HDG acts as a primer for the IFRM system and prevents the occurrence of under film corrosion when IFRM begins to deteriorate and expose the galvanized surface (consult Duplex systems: Hot-Dip Galvanizing Plus Painting by J.F.H. van Eijnsbergen, published in 1994). The combined effect results in maintenance-free corrosion protection for 1.5 to 2.3 times the sum of the IFRM service life and HDG time-to-first maintenance. Further, primers applied prior to the intumescent paint layers will last 50 per cent longer in service life when applied as a tie coat over a HDG surface in comparison to when used as an IFRM primer applied over bare steel (consult Duplex systems: Hot-Dip Galvanizing Plus Painting by J.F.H. van Eijnsbergen, published in 1994).
To achieve these benefits when applying IFRM coatings over hot-dip galvanized coatings, the tie coat specified for application over the galvanized surface must be approved by the IFRM manufacturer. This ensures minimum specified bond-strength requirements are met and the IFRM will perform as intended. For example, oil-based tie coats comprising alkyd and epoxy esters, or linseed oil derivatives should be avoided as these paints can saponify and eventually delaminate when applied over the alkaline hot-dip galvanized surface. On the other hand, if the tie coat is incompatible with the intumescent paint, the latter will either delaminate or develop large holes or unprotected areas in the coating when exposed to elevated temperatures (consult “Galvanizing, Primer Adhesion Test and Beyond” by Ernst Toussaint, published in 2019). To avoid these issues, passive fireproofing manufacturers can provide a list of approved tie-coat products tested for compatibility with the IFRM. Where an approved tie coat is not readily available or where field confirmation is required, bond testing and small-scale fire testing can be performed independently.
Preparing HDG surfaces for passive fireproofing application
As with any industrial coating application, proper surface preparation is key to the overall success of passive fireproofing materials applied over a galvanized surface. However, determining the steps for surface preparation can be challenging as these requirements vary by type of fire-resistive material and can further vary by manufacturer or product line. Some passive fireproofing materials (i.e. rigid board and flexible blanket systems) are mechanically fastened to structural steel, and, therefore, no specialized preparation of the hot-dip galvanized coating surface is required prior to installation. On the other hand, preparing galvanized surfaces for SFRMs and IFRMs often require additional preparations to promote adhesion, achieve the specified cohesive/adhesive bond strength requirements, and ensure fire-resistive coatings perform as intended while avoiding damage to the galvanized coating.
Cementitious and gypsum-based SFRMs require the removal of oils, grease, dirt, and debris from the HDG surface at a minimum (read the National Fireproofing Contractors Association’s (NFCA’s), “NFCA Standard Practice 200 Field Quality Assurance Procedure for Application of Spray Applied Fire-Resistive Materials (SFRMs)”, published in 2004, and refer to “Fire-resistance Ratings,” the American National Standards Institute/Underwriters Laboratories (ANSI/UL) 263-2018, Standard for Fire Tests of Building Construction and Materials). For such materials, solvent cleaning per Society for Protective Coatings (SSPC)-SP1, Solvent Cleaning, should provide sufficient preparation. Depending on the structural member dimensions and the results of bond tests conducted in accordance with ASTM E736, Standard Test Method for Cohesion/Adhesion of Sprayed Fire Resistive Materials Applied to Structural Members, additional preparations can be employed to promote the adhesion of SFRMs over HDG. Additional recommendations prior to application of the fire-resistive material may include:
Occasionally, failures in lightweight cementitious fireproofing have occurred when a bare metal lath is used. As the bare metal eventually corrodes, the voluminous iron oxide (Fe2O3) corrosion causes a buildup of pressure that can result in cracking in fireproofing. Where metal lath is required, the use of galvanized mesh is recommended to maximize the longevity of the fireproofing and minimize the chance for accelerated corrosion under fireproofing (refer to “Corrosion of Galvanized Steel Under Fireproofing” by A.M. Stoneman and F.E. Goodwin).
If a tie coat or bonding agent is required, the galvanized surface should be prepared in accordance with the recommendations provided in ASTM D6386, Standard Practice for Preparation of Zinc (Hot-dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting. Depending on the initial surface condition of the galvanized coating (new or partially or fully weathered), ASTM D6386 describes different levels of cleaning required to improve the bond of a tie coat or bonding agent to the zinc surface.
ASTM D6386 also addresses a variety of post-galvanizing treatments and surface conditions that must be addressed prior to tie coat application. First, the galvanizer must be informed to avoid quenching or passivation of the parts after galvanizing. Although these post treatments assist the galvanizer in handling the parts at the plant, the result is often a surface, which is difficult to apply a top coat (read the 2015 publication “Duplex Systems: Painting Over Hot-Dip Galvanized Steel” by the AGA). Next, there are occasionally surface conditions present on hot-dip galvanized coatings which do not affect corrosion protection and are considered acceptable (e.g. roughness, runs, small inclusions, or bumps), but these same conditions negatively affect the adhesion and performance of tie coats (Figure 3). ASTM D6386 contains a list of these surface conditions that must be remedied or smoothed prior to cleaning the galvanized surface.
After surface smoothing, the galvanized surface must be roughened in accordance with the tie coat manufacturer’s recommendations. ASTM D6386 lists various methods which can be used including:
However, sweep blasting in accordance with SSPC SP16, Brush-Off Blast Cleaning Non-Ferrous Metals, is the most common method of roughening the galvanized surface without damaging the coating. If the tie coat applicator is unfamiliar with preparing hot-dip galvanized surfaces, they may unintentionally use another abrasive blast cleaning standard intended for cleaning bare steel. If SSPC SP16 is not explicitly specified, over blasting may occur and result in peeling or excessive removal of the hot-dip galvanized coating (Figure 4) (consult the white paper “Corrosion Under Insulation” by MTI, published in 2012).
IFRMs require preparation and application methods similar to those used for conventional coating because they typically comprise an acrylic, vinyl, or epoxy primer coat directly applied to structural steel prior to application of intumescent paint layers and a sealer/top coat. Similar to the preparation of tie coats for SFRMs, IFRM tie coats should also be applied to a galvanized surface prepared in accordance with ASTM D6386.
After surface preparation of the galvanized surface has been performed, it is recommended to refer to the manufacturer’s instructions regarding any application of bonding agents, metal lath, tie coats, subsequent fireproofing layers, or sealants comprising the overall fireproofing system. Additional application requirements will be outlined in the certifications directory listing from the recognized authority where the fireproofing system was evaluated and approved such as the Underwriters Laboratories (UL), Intertek, Factory Mutual (FM) Approvals, etc.
HDG is capable of providing building projects with long-term corrosion protection and extending the steel’s life for a variety of passive fire-proofing systems. However, the key to unlocking these benefits is to ensure the galvanized surface is prepared suitably for the specific fire-resistive product and in accordance with the manufacturers’ recommendations. With the added benefits of HDG, fireproofed structural building members can give rise to safe and corrosion-resistant building structure that will stand the test of time and be enjoyed for generations.
Alana Hochstein is the senior corrosion engineer for the American Galvanizers Association (AGA). Hochstein provides assistance to architects, engineers, fabricators, owners, and other specifiers regarding technical issues and the processing of hot-dip galvanized steel. She also manages AGA studies and research on performance, application, and processing of hot-dip galvanized steel. Hochstein can be reached via e-mail at email@example.com.
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