• Introduction

Office buildings have many water systems that demand constant attention.  Potable cold and hot water, fire water, chilled water, and cooling water all must be monitored and tended at some point.

            Chilled water is a closed, re-circulated system, and is inhibited to protect steel surfaces from internal corrosion.  Cooling water is a re-circulated system, and treated to protect internal piping surfaces and cooling towers.  Potable water for drinking, sanitation, irrigation, sprinklers, and fire water supply, on the other hand, are supplied by the city.  This water supply is filtered, chlorinated and fluorinated then supplied throughout the city system.  All soluble salts indigenous to the area remain in the water.  Under the right conditions, the common water supplied by the city, which we take for granted, can cause serious corrosion related issues. 

• House Tank Corrosion

The problem with the potable water starts at the house tank, where city water is introduced into the building system.  The filling process is by an open flow line into an atmospheric, non-pressurized steel tank.  This filling process can cause oxygen enrichment.  Time in the open tank will cause chlorine depletion.  From the tank, the potable water is pumped to all floors in the building and as makeup to the hot water boiler and circulating system.  

Oxidation of the tank interior surface, in the form of rust, progresses below the water line and in the wet vapor phase in the top area of the tank.  Common oxidation is usually a slow process, but in potable water tanks, more aggressive chemical reactions take place.  The most common and most damaging to the steel tank internal surface is microbiological attack.  Microbes are present in potable water.  They are not harmful to humans but, in the water, attach to the steel surface and flourish under the protection of a rust film.   

            The microbes are called anaerobic corrosive bacteria.  They create localized environments by secreting corrosive acid wastes.  They are always found under other deposits in oxygen deficient locations such as under rust, in crevices, and in the bottom of pits.  Many bacteria create a mucilaginous substance that encapsulates the cells, shielding them from direct contact with water.¹  

¹ (The Nalco Water Handbook, Nalco Chemical Company, McGraw-Hill Book Company 1979, Page 22-2) 

            Hidden from chlorine control and wrapped in their protective jackets, the bacteria can flourish.  As rust and pitting and oxidation spread over the internal surface, bacteria and the resulting acid formation increases.  The aggressive attack on the steel accelerates and the steel walls and floor of the tank will soon fail.  It follows that a smooth, slick lining surface offers no opportunity for bacteria to contact the steel and no oxygen deficient, hidden sites in which bacteria can grow and multiply.   

Microbiological conditions conducive to corrosion can be visually observed.  The steel surface under the water is covered with a black, sooty material that, when rubbed, leaves the black soot on the fingers.  Most tank interior surfaces having heavy incrustation of rust also has evidence of the microbes.

It is obvious that interior tank protection is very important.  Many tanks are lined before installation, when the building is new.  Unfortunately, many of the linings fail, either because an inappropriate epoxy system was selected, the surface was not properly prepared, or the application was not adequate.  Most lining materials will absorb water slowly over the years and result in film failure or adhesion failure.  This happens because the epoxy molecules are not sufficiently cross-linked or they de-polymerize with time.   Lining materials that are to protect the tank for many years must be very dense and stable and completely unaffected by the years of water immersion. 

            Visual observations of the lining in a tank can present clues to the cause of failure.  Blisters are caused by absorption of water into the film, sometimes altering the molecular structure of the polymer.  The lining film must expand and soften in order to form blisters.  In adhesion loss, the rigid film pulls away from the surface in an unaltered state.  This is caused by water permeating through the lining and reacting at the surface. Poor initial wetting out of the lining material may also cause adhesion loss during application.   The poor wetting can be a fault of poor lining material selection or poor surface preparation.

Corrosion on the steel surface above the waterline is also aggressive in that the atmosphere is usually oxygen rich and very humid.  This is the ideal environment for rapid oxidation.  Thus, the vapor phase in the top of a tank must be carefully lined.

            A successful lining system in a house tank must remain in place for many years, being impervious to water attack and water absorption.  It must also resist attack by chlorine, fluorine, and the many dissolved salts present in city-supplied water. When a tank is re-lined in place, the lining material must have a number of special properties: 

·         Speed of surface cleaning, lining application and lining cure must be short. Typically the water system in a building cannot be out of service for long.  

·         Solvents and other organic vapors cannot be released into the atmosphere to cause an odor or otherwise degrade the air in the building. Usually the house tank is within the building environment.  

·         The lining material must have excellent wetting out and adhesion properties.  Internal surface preparation methods are limited because of time, space, and environment limitations.  

·         The cure time must be short.  The water supply tank must be placed back in service in as short a time as possible.

·         The lining material must be able to continue cure under water.  Time does not allow for full cure before filling of the tank.

·         The lining material, both in liquid and cured phase, must be NSF approved for food/water contact.  The lining material can impart no odor or taste to the water.

• Fire Water System Corrosion

Firewater tanks contain the same water as the house potable water tank.  Many times the tank is shared, with the house supply water serving as fire water supply if needed.  If a building has a dedicated firewater tank, the internal surface must be protected with a lining system just as in the case of the potable water supply tank.  All issues are the same.

• Internal Pipe Corrosion Failure

Internal pipe corrosion is common in both the coldwater distribution piping and the hot water circulating system.  If a pinhole leak occurs anywhere in the piping system, it is important to determine the cause immediately.  The one leak usually means that there are many more in various stages of development.  

Piping leaks from internal corrosion are usually the result of: electrochemical attack caused by dissimilar metals in the system, erosion pitting caused by turbulent flow of water having suspended solid particles, or microbiological pitting attack.  By removing and splitting sections of pipe from suspected areas, the cause for failure can be determined and appropriate remedial action taken.  But, action must be taken quickly if total replacement of the pipe system is to be avoided.

• Conclusions

The management of potable water must be recognized as an important issue.  Protection of internal surfaces of the house tanks and piping system cannot be ignored.  Early detection and remediation can save money and downtime.  The choice of the right material and application contractor, together as a unit, is vital.  Choose a supplier with proven successes in both new applications and remediation work.  Leaky tanks in the poorest of conditions can often be brought back to like-new conditions, with the right choices.

E. Don McCrory, PE

An engineering graduate of the University of Notre Dame, Mr. McCrory has been directly involved in internal corrosion for 42 years, chairing numerous NACE committees in the internal lining and protective coating fields.  This knowledge has been put to use in service to the commercial building industry through McCrory Engineering, Inc., which he founded in 1979