Hot-dip galvanized steel is often utilized in some of the harshest environments imaginable, yet it provides maintenance-free longevity for decades. The corrosion resistance of hot-dip galvanizing varies according to its surrounding environments but generally corrodes at a rate of 1/30 of bare steel in the same environment. Measurements of the actual consumption rate of the coating during the first few years of service provide good data for projecting a conservative estimate for the remaining life to first maintenance.
When embedded in concrete, hot-dip galvanized steel can withstand the different corrosive elements and fulfill the intended design life. More information about hot-dip galvanized steels longevity in various environments can be found in the AGAs publication Performance of Hot-Dip Galvanized Steel Products.
Zincs Initial Reaction in Fresh Concrete
During curing, the galvanized surface of steel reinforcement reacts with the alkaline cement paste to form stable, insoluble zinc salts accompanied by hydrogen evolution. This has raised the concern of the possibility of steel embrittlement due to hydrogen absorption. Laboratory studies indicate liberated hydrogen does not permeate the galvanized coating to the underlying steel and the reaction ceases as soon as the concrete hardens.
ASTM A767 requires hot-dip galvanized reinforcement be chromate passivated after galvanizing. Many cement mixtures contain small amounts of chromate that may serve the same purpose as chromate passivating the zinc coating. The reaction between the alkaline cement paste and the zinc coating is dependent on the amount of zinc-coated surface in the concrete with the potential for reaction increasing with more zinc metal in contact with the concrete.
Zinc reacts with wet concrete to form calcium hydroxyzincate accompanied by the evolution of hydrogen. This corrosion product is insoluble and protective of the underlying zinc (provided that the surrounding concrete mixture is below a pH of about 13.3). Research has shown that during this initial reaction period until coating passivation and concrete hardening occurs, some of the pure zinc layer of the coating is dissolved. However, this initial reaction ceases once the concrete hardens and the hydroxyzincate coating has formed. Studies of galvanized rebar recovered from field structures indicate that the coating remains in this passive state for extended periods of time, even when exposed to high chloride levels in the surrounding concrete.
For concretes of high pH, or where some background chlorides are expected, the zinc surface can be passivated, using a range of proprietary post treatments, as a safeguard against excessive hydrogen evolution that may, in serious cases, reduce the pullout strength of the bar. For normal concrete conditions, research has shown no statistical difference in bond strength between galvanized rebar that was passivated and not passivated.
Concrete is an extremely complex material. The use of various types of concrete in construction has made the chemical, physical, and mechanical properties of concrete and its relationship to metals a topic of ongoing studies. Reinforcing steel bars (rebar) are embedded in concrete to provide strength and are critical to the integrity and performance of the structure throughout its life. As concrete is a porous material, corrosive elements such as water, chloride ions, oxygen, carbon dioxide, and other gases travel into the concrete matrix, eventually reaching the rebar. Once the concentration of these corrosive elements surpasses steels corrosion threshold, the rebar starts to corrode. As the rebar corrodes, pressure builds around the bar leading to cracking, staining, and eventually spalling of the concrete.
Because failure of the rebar leads to compromised or failing structural capacity, protecting against premature rebar failure is key. Similar to in the atmosphere, galvanized rebar extends the life of the steel in concrete. The corrosion mechanisms in concrete are quite different than atmospheric exposure, and one of the biggest factors is chloride concentration. Galvanized rebar can withstand chloride concentration at least four to five times higher than black steel, and remains passivated at lower pH levels, slowing the rate of corrosion. In addition to the higher chloride tolerance, once zinc corrosion products are formed from the galvanized rebar, they are less voluminous than iron oxide and actually migrate away from the bar. The less voluminous zinc particles migrate away from the bar (galvanized coating) and into the pores of the concrete matrix. This migration prevents the pressure buildup and spalling caused by iron oxide particles.
The total life of galvanized steel in concrete is made up of the time taken for the zinc to depassivate, plus the time taken for consumption of the zinc coating, as it sacrificially protects the underlying steel. Only after the coating has been fully consumed in a region of the bar will localized steel corrosion begin. Laboratory data support, and field test results confirm, that reinforced concrete structures exposed to aggressive environments have a substantially longer service life when galvanized rebar is used as opposed to bare steel rebar.
Rebar in Concrete Corrosion Model
The total life of a galvanized coating in concrete is made up of the time taken for the zinc to depassivate (which takes longer than black steel because of its higher tolerance to chloride ions), plus the time taken for the consumption of the zinc coating as it sacrificially protects the underlying steel. Only after the coating has been fully consumed in a region of the bar will localized corrosion of the steel begin.
Black and galvanized rebar corrosion performance in concrete are shown in the graph above. As you can see, zinc coatings have a higher chloride (Cl -) corrosion threshold (2-4 times) than that of uncoated steel, significantly extending the time until corrosion initiation. Once corrosion of the zinc coating does occur, the properties of the corrosion products and their ability to migrate into the concrete matrix reduces stress generation in the surrounding concrete, further extending the life of the reinforced concrete structure.
Efforts have been made in many zinc-coated steel applications to develop the correct test method to determine a proper accelerated lifetime. One test for corrosion prevention systems in the United States is ASTM B117. ASTM Committee G-1 on Corrosion of Metals has jurisdiction over the salt spray standards B117 and G85. The Committee passed the following resolution regarding the use of B117: ASTM Committee G-1 on the Corrosion of Metals confirms that results of salt spray (fog) tests, run according to ASTM standard designation B117, seldom correlate with performance in natural environments. Therefore, the Committee recommends that the test not be used or referenced in other standards for that purpose, unless appropriate corroborating long-term atmospheric exposures have been conducted. ASTM B117 and B368 are best used as quality control tests assuring that the day-to-day quality of products and manufacturing processes are optimized. There are a number of other corrosion tests which can be used for predicting performance in service.
Salt spray tests cannot be used to accurately test zinc-coated steel because they accelerate the wrong failure mechanism. Without a proper wet/dry cycle, the zinc coating cannot form patina layers. The absence of a patina layer allows constant attack of the zinc metal and gives a very low prediction of the zinc coating lifetime. Core testing has been shown to provide the most accurate results for the performance of galvanized rebar embedded in concrete.
Longevity Case Study
Athens Bridge- Athens, PA, 1973
The Pennsylvania DOT has specified galvanized reinforcement for decades. One such bridge, the Athens Bridge, built in 1973, is an eleven-span, four-lane, divided bridge utilizing hot-dip galvanized reinforcing bars. The Athens bridge deck was initially inspected eight years after installation. Concrete cores were drilled and an analysis of chloride contamination and coating thickness was conducted. The chloride levels found in the cores exhibited concentrations between 1.8 to 7.9 lbs/yd3 of concrete. The high end of these concentrations is well above the threshold for active corrosion to occur on bare steel. Despite these extremely corrosive conditions, the coating thickness measurements indicated galvanized coatings in excess of 15 mils (approximately three times the coating thickness required on newly-galvanized rebar according to ASTM A 767).
The Athens Bridge was later inspected in 1991 and 2001, and the analysis generated similar results. No sign of active corrosion on the galvanized reinforcement was found and coating thickness measurements reported were in excess of 10 mils. These current coating thicknesses indicate an estimated 40-plus years of additional maintenance-free corrosion protection.