One of the fundamental questions confronting power generation plant maintenance engineers and managers is not what corrosion protection system to specify but how much money to invest upfront to minimize or eliminate recurring maintenance. Possible answers include: do nothing, paint or hot-dip galvanize. Analyzing all three alternatives should involve not only a performance evaluation, but initial and life-cycle cost analyses. Although it is not a new solution to corrosion protection, hot-dip galvanizing has proven its mettle.
Zinc has been used to hot-dip galvanized steel for 250 years and has been delivering protection from corrosion in harsh, corrosive environments for 75 years or more. The empirical data collected from the field performance of hot-dip galvanized steel from 1940 to 2005, in industrial and manufacturing settings, indicates that zinc prevents corrosion of the base steel far beyond other surface treatments. This feature means that Powder River Basin (PRB) or eastern coal-fired plants benefit from using galvanized steel for conveyors, vibratory feeders, coal hoppers, chutes, columns, girders, trusses, steps, stringers, handrails, grating and expanded metal because maintenance costs are essentially eliminated.
Since 1920, the American Society of Testing Materials (ASTM) and the American Galvanizers Association have measured and collected data on the corrosiveness of various atmospheres and micro-environments. Hot-dip galvanized coatings perform without maintenance for 50 years or more in industrial environments, including coal-fired power plants (Figure 1).
How Does Hot-Dip Galvanized Steel Do It?
Hot-dip galvanized steel provides three-way protection from corrosion:
- Cathodic. Zinc is more anodic than steel. Thus, when a corrosion cell forms-for example, when both an electrolyte and return current path are present in the zinc and steel-the zinc readily gives up electrons to protect the steel from corrosion. Zinc will protect the base steel until all of the galvanized coating is consumed.
- Barrier. Zinc is impermeable and does not allow moisture (electrolytes) to penetrate the galvanized coating. Thus, the base steel is protected by the impervious barrier.
- Patina. When exposed to the atmosphere immediately after completing the galvanizing process, zinc reacts with oxygen in the air to form a thin zinc-oxide powder on the galvanized coating surface. After a few days, the zinc-oxide reacts with water molecules in the air to form zinc-hydroxide. Over a period of months, as the zinc-hydroxide is exposed to airborne carbon dioxide, a thin film of zinc-carbonate forms. Zinc-carbonate, a passive patina film tightly bound to the galvanized coating, gives a hot-dip galvanized coating its durability.
Conveyor and chute steel abrasion caused by contact with coal is also a significant issue in coal plants. Using hot-dip galvanized steel can eliminate this issue because it is difficult for the coal to damage the coating as the zinc-iron alloy layers are harder than the substrate steel.
Although initial cost is a critical factor that must be considered when selecting the correct corrosion protection system, the long-term maintenance costs should also be regarded before making the final decision. Quantifying the life-cycle costs for hot-dip galvanizing is quite simple, but for most barrier protection systems, it can be a daunting task, especially when the time-value of money is calculated. The following example examines the initial costs for a hypothetical power plant for hot-dip galvanizing and for a three-coat paint system that is often used as an alternative for corrosion protection.
“Unnamed” Powder River Basin Coal-Fired Power Plant
- The power plant contains 6,000 tons of structural steel, including conveyors, chutes, hoppers, walkways, handrail, platforms, columns and girders.
- The typical mix is 250 square feet of steel per ton.
- The facility’s projected service life is 45 years.
- The plant burns low-sulfur, micro-environment coal (C5-I industrial environment of high humidity and aggressive atmosphere, according to ISO 12944-2 “Classification of Environments”).
An initially less expensive and a less durable paint system can be selected, but when the life-cycle cost is calculated (maintenance costs are imputed over the 45-year projected life of the coal-fired power generating facility), the selection of hot-dip galvanizing is clearly superior. Table 1 shows that the initial cost of hot-dip galvanizing is basically the only cost because the final or life-cycle cost is negligible since hot-dip galvanizing has been documented to last approximately 75 years in an industrial environment. The recommended maintenance practice for the three-coat paint system includes touchup, maintenance and full repaint, which results in the paint system’s long-term costs being five to 10 times greater than the hot-dip galvanizing costs.
An AES Power Plant in San Juan, Puerto Rico, with two coal-fired boilers, was built in 2002 and contains 10,000 tons of hot-dip galvanized steel. Timely, weather-independent steel delivery staged for just-in-time delivery was a key decision variable, but the life-cycle cost clinched the deal for galvanizing. The galvanized steel’s life-cycle cost was 84 percent less than a two-coat paint system. Maintenance-free service is expected for decades.
The McDuffie Coal Terminal in Mobile, Ala., was constructed in 1995 and contains 2,000 tons of hot-dip galvanized steel used for conveyors, structural steel, frames, walkways and handrails. Hot-dip galvanized steel was selected because of the abrasion resistance and durability that virtually eliminates maintenance to the conveyor. When the hot-dip galvanized steel was inspected in 2006, after 10 years of service, it still looked like new, and is predicted to remain in service for 40 or more years.
Coal-fired power plant maintenance engineers can have the best of both worlds if they elect to use hot-dip galvanized steel for plant construction. They can have an economical corrosion protection system on an initial cost basis and one that is durable enough to deliver maintenance-free performance for the life of the facility. In addition, the corrosion kinetics of cathodic, barrier and patina protection that hot-dip galvanized steel provides make it suitable for a PRB plant’s corrosive atmosphere.