In Moisture-Rich Conditions
A less common environment for steel in the power sector is submerged in or exposed to water. Moisture is highly corrosive to most metals including steel and zinc. However, because of the development of the passive, impervious zinc patina, the corrosion rate of galvanized steel is much slower than bare steel. There are many different types of water (pure water, natural fresh water, potable water (treated drinking water), and seawater) and each has different mechanisms that determine the corrosion rate.
Similar to soils, the varieties of water make predicting corrosion rates difficult. Though pH level has the most profound effect, many parameters affect corrosion of metals in a water environment including oxygen content, water temperature, agitation, the presence of inhibitors, and tide conditions. Despite the difficulty of predicting corrosion, hot-dip galvanizing steel is one of the best methods of corrosion protection for submersed applications because of its complete, uniform coverage.
The most obvious water application in the power sector is in hydro-electric generation plants. Hydro-electric plants are designed with a life expectancy of several decades, sometimes even longer than coal-fired facilities. Though the initial investment in a hydro facility is daunting, studies show a dam can recoup the cost of construction after 5-8 years of generation at full capacity. To capitalize on this early return on investment, it is important to utilize low maintenance materials to avoid costly shutdowns and replacement costs.
As hydro-electric facilities are most often located in freshwater lakes, reservoirs, and rivers, galvanized steels performance in fresh water is most critical. As many of the transfer pipes and penstocks exist submerged in water, they are constantly exposed to the corrosive attacks of moisture. Galvanized steel performs very well under these conditions, performing flawlessly even in harsh water environments such as seawater for 8 12 years.
Longevity Case Study
Candelabra Tower - Miami, FL 2009
Piercing the cloudy heights of the Miami sky, the Candelabra communications tower is the tallest structure in the city at 1,042 feet. The structured tangle of more than 460 tons of tubular members, solid bar leg structures, fasteners, anchor bolts, and angle bracing was so impressive, it was featured in the October 2009 issue of Modern Steel Construction highlighting the challenging size and design of the project. A structure of this size had to slip-fit and all bolted holes needed to align and be clean and useful at the extreme height the material was to be assembled (more than 1,000 feet).
With the height of Candelabra, corrosion could be structurally devastating and the highly corrosive saltwater environment of Miami meant every precaution must be taken to protect the steel from the potential threat. For more than a decade, the owner has chosen to hot-dip galvanize their radio and communication towers because of the consistently proven durability for this application. Given this experience, the owner felt there was no better way to combat the corrosive sea air and seasonal storms typical to the city than to hot-dip galvanize all of the critical structural components. Galvanized steel is the most effective means of protection against the harsh sun, rain, and salty sea air of Miami. Specifiers in southern Florida now have a structure to inspect and examine in the future when determining what corrosion protection system to utilize, as the Candelabra will stand head and shoulders above the rest, corrosion-free, for generations to come.