POWER April 2013 - 14

Battling White Rust
Does your power plant use a chiller for combustion turbine inlet
air cooling or other processes that reject heat? If so, there
is a good chance you also have an auxiliary cooling tower
(Figure 1) or a wet surface air cooler (WSAC, Figure 2) to cool
these systems. (See POWER, September 2008, " Wet Surface
Air Coolers Minimize Water Use by Maximizing Heat Transfer
Efficiency. " ) Galvanized steel has been commonly used in the
construction of smaller, package-type cooling tower systems
because it is relatively inexpensive. These cooling towers are
popular in many different industries and will increase in use
in power applications.
Those of us who work in the water treatment industry are
familiar with the smaller, package-type cooling towers from
our experience in the food and beverage, institutional, and
manufacturing business units. These units are typically constructed
using galvanized and stainless metallurgies. However,
many in the power industry are only familiar with wood
or fiberglass-reinforced plastic (FRP) because these are the
materials most commonly used in the construction of fielderected
cooling towers at central station power plants.
1. Cool breeze. This is a typical auxiliary evaporative cooling tower.
Courtesy: Nalco Power ITC
A cooling water treatment program must consider not just
the galvanized, wood, or FRP but also all the materials that
the water will come into contact with throughout the plant,
such as titanium, stainless steel, and copper-based alloys
used in the condenser and possibly other balance-of-plant
heat exchangers.
The question of acceptable chloride levels in the bulk cooling
water always comes up anytime stainless steel alloys are
used. Typically, the engineering firm responsible for the plant
design will select appropriate materials and a water chemistry
program that ensure chloride levels in the cooling water are
kept at acceptable levels, depending on the grade of stainless.
When stainless steel is used, developing an acceptable
water treatment program is straightforward.
Galvanized cooling towers are a much different story. Although
rare in the power industry, towers using galvanized
steel components are becoming more popular with the increased
use of chillers for inlet air-cooling combustion
turbines in combined cycle plants, as well as WSACs in the
air-cooled configuration.
In general, all cooling towers are very sensitive to water
chemistry, particularly chlorides, sulfates, and alkalinity.
These constituents are often overlooked when designing a
cooling water treatment program. (See POWER March 2013,
" Selecting a Combined Cycle Water Chemistry Program " and
September 2009, " Avoid These 10 Mistakes When Selecting
Your New Water Treatment System. " ) Even so, using galvanized
parts within a cooling tower makes the problem of water
treatment even more difficult-the water is tough on the
galvanized parts, and the power industry operates cooling
towers at much higher cycles of concentration than other
industries.
Finally, the propensity for white rust formation in galvanized
systems is not very well known in the power industry.
The remainder of this article will define white rust, how it
is formed, its potential impacts on equipment integrity, and
how to control its formation.
2. Go with the flow. The co-current flow of air and water (both
flow downward) in a wet surface air cooler increases the system's heat
transfer efficiency. Courtesy: Nalco Power ITC
Galvanized Steel and White Rust
Galvanized steel is carbon steel with a coating of zinc metal
on its surface. Either a hot-dip or electroplating process can
produce a zinc layer that is a protective surface coating as
well as a sacrificial anode. Because zinc is less noble than
carbon steel, it will corrode sacrificially to protect the underlying
base metal. If the protective zinc layer should corrode
through, the base-layer steel will corrode at the same rate as
unprotected carbon steel. It is important to note that galvanized
steel will always have some small amount of general
corrosion occurring, just as with mild steel. The key is to
minimize those general corrosion rates while minimizing the
potential for localized corrosion.
When a galvanized steel system is operated under nonaggressive
conditions such as a neutral pH and moderately hard
water, a protective layer of nonporous zinc carbonate and
zinc hydroxide forms to prevent further corrosion of the zinc
coating. However, if the system is operated under aggressive
conditions such as high pH, low hardness, or high chloride or
sulfate levels, rapid corrosion of the protective zinc layer can
occur. If the corrosion is severe, all the galvanized coating
14
www.powermag.com
POWER | April 2013
http://www.powermag.com
POWER April 2013

Table of Contents for the Digital Edition of POWER April 2013
