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Tank Failures

Steel cold domestic water storage tanks, often called house tanks, are generally viewed as low maintenance, long service life structures necessary to most larger building properties and some process plants.  The taller or larger the property, the more tanks which may exist, and larger in size.  In addition to providing a cold water supply at the appropriate pressures, they often also provide reserve water for fire protection, and range in size from 2,000 gallons to 60,000 gallons and greater.

Those used for potable water storage, by law and local codes, must be regularly cleaned and sterilized on the inside by licensed professionals in order to satisfy health concerns, although often remain neglected.  Tanks situated in moderate to high traffic areas typically receive a coat of paint on the exterior of their exposed and accessible surfaces and are well maintained.  Those tanks which are hidden away in basements, roof areas, and corner storage rooms, not surprisingly, are often ignored.

On the inside, corrosion protection is achieved strictly by placing a boundary or isolating coating between the water and the steel plate.  Annual inspection and cleaning, if performed properly, generally raises notice of any coating deficiency, and provides the opportunity to make repairs before significant damage occurs.

• Interior Wall Loss

A common area of interior wall loss occurs at the area of the water line.  Here, constant washing back and forth over the same area tends to fail many protective coatings, and then heavily damages the underlying steel.  The below sample graphs, created after performing 500 or more ultrasonic measurements per side wall, well illustrates not only this particular problem, but the power of ultrasound to produce a virtual 3-D dimensional profile of each interior tank wall and bottom.

At the below left graph, ultrasonic testing along a standard X-Y grid at the tank side walls produces a clear picture showing a failure of the coating and the resulting corrosion at the water line.  We see the uniform wall thickness profile above the water line in the center, with high corrosion and pitting at the side walls as shown by the spikes.

At the right side graph, based upon a separate ultrasonic investigation, we can show a more uniform wall thickness at the front and back heads.  However a few areas of random deep pitting exist in the rear head ( right side of graph ).  The front head shows no areas of deep pitting, however a globally lower wall thickness exists here due to the constant movement of the water inlet located at the front head ( left side of graph ).

• Cold Surface Condensation

The same maintenance level is rarely assigned to the outer tank surface, however, which often results in metal deterioration ranging from mild to severe.  Exterior steel tank deterioration is due to a variety of physical factors – water temperature, tank temperature, tank size and location, construction, area humidity, coating effectiveness, tank supports, and physical accessibility.  While interior coatings do provide generally good corrosion protection, their periodic failure often results in localized areas of wall loss.  Exterior wall loss, however, can equal and often exceed that occurring on the inside.  Although plainly visible, it is often neglected as a maintenance item.

For larger tanks, the cold inlet water will often settle or stratify to the bottom rather than mix to produce a uniformly moderate tank temperature – therefore it is quite common to find a significant temperature gradient between top and bottom.  Given sufficient humidity, such as is typically found in a mechanical room area, surface condensation can develop to the point of producing a moisture boundary layer, and even water droplets, at the tank’s bottom and lower sides.

Condensation, and the resulting corrosion it produces, is not only far more prevalent at the bottom of the tank due to the colder surface temperatures, but produces the greatest damage in that area simply due to the fact that it is outside of normal view.  It is almost unheard of to find the underside of any steel tank, and its supports painted or in any way protected – thereby allowing corrosion free travel if conditions permit.

Condensation collected onto outer tank walls can similarly produce a corrosion problem, but to a lesser degree if it is noticed and corrected by building maintenance personnel.  Most side wall corrosion problems exist where tanks are placed directly against one or more walls.  In many cases, tanks are constructed only a few inches away from two or more walls, often because they are built in place, and thereby prevent any maintenance or inspection from being performed.

• Painting No Guarantee

Given sufficient time, and a protective coating which is not uniform, not well adhered to the metal, or having damaged areas or entry points, moisture will eventually deteriorate most steel tank walls.  In most examples, standard industrial paints are used where specialty coatings are actually required.

With an entry point established, moisture will condense beneath the protective coating and gradually work its way under the coating to produce a blistering of the tank surface.  In some cases, as much as a cup of water has been found trapped underneath a single coating blister, and against the bare steel surface.  This corrosion mechanism, however, is a very long term process – typically requiring decades to fail a steel tank.

Exterior corrosion is especially common at older riveted steel tanks having irregular surfaces where the paint or coating may not be of uniform thickness.  In many cases, blistering can extend over wide areas, producing a deeply pitted surface due to direct contact with what is essentially untreated water.

Often, the greatest damage caused to the bottom of a house tank is not at the tank surface, but at the tank to I beam supports.  Here, a combination of moisture, no corrosion protection, and mild galvanic activity between tank and beam can actually undermine the tank bottom.  Since steel expands as it corrodes, rust produced in this area can actually lift a 40,000 gallon water storage tank 1 inch or more.

A variety of common problems found at house tanks is provided in the below photo gallery.

Different Forms Of Tank Corrosion

Riveted Seams – Commonly found at very old tanks installed before the 1950’s, riveted seams can be a frequent source of problem. Due to the low pressures involved, only one set of rivets typically attaches overlapping steel plates – thereby providing less sealing strength and greater chance for corrosion to occur and a leak to begin.	Cold Water Settling – Cold water generally settles to the bottom area of any large house tank to produce this result.  Insulation is almost never provided. Where the steel is lacking adequate corrosion protection, some degree of corrosion and pitting usually results.  Metal loss at the bottom of the tank is usually the most severe.

Maintenance Neglect – This domestic water house tank shows clear evidence that it is in need of sandblasting and the application of a good quality protective coating. Patchwork repair is not likely to provide acceptable corrosion protection since other problems areas will develop.	Bottom Corrosion – This represents a most difficult problem area since substantial corrosion product often accumulates between the tank bottom and tank support base. A penetrating chemical rust inhibitor would be suggested.

• Levels Of Inspection

A preliminary inspection for exterior water tank corrosion can be usually performed in-house by carefully looking over the entire structure, top to bottom, and at every side.  Special attention, of course, should be provided to the bottom of the tank, at I beam supports, and at any areas which are generally out of view.

A higher level of tank inspection involves more than just a visual examination for surface rust, and typically utilizes ultrasonic testing to identify remaining tank thickness and ideally – a tank wall thickness profile.  An analysis based upon the results of an ultrasound test will generally follow API inspection specification 650, and take into account other physical factors such as:

• Tank shape and size
• Tank supports and spacing
• Tank construction – riveted or welded
• External corrosion
• Tank to support corrosion
• Wall thickness profile
• Minimum acceptable wall thickness limit
• Corrosion rate
• Construction joint efficiency
• Corrosion trends
• Remaining service life
• Percentage of allowable loss
• Overall condition or status

• Difficult Repairs

The repair of such external corrosion problems generally depends upon the physical location of the corrosion activity.  In accessible areas such as side walls, it is only necessary to thoroughly scrape away the corrosion deposit and old coating, and re-coat the surface.

Where damage has already occurred and the surface is heavily pitted, we recommend sand blasting prior to applying any coating, as only sand blasting can be assured of producing a reliable metal to coating bond.  In humid environments, specialized coatings suitable for marine use would be recommended instead of common industrial paints.

Tank bottoms always demand the most difficult repair – especially where corrosion exists at the tank to I beam supports.  Here, the corrosion product may be to difficult to remove, metal damage may be too advanced to simply paint over, and major reconstruction may be necessary in order to replace the tank support lost due to corrosion.

Adding I beam supports maybe necessary due either to the loss of bottom wall thickness, or due to a weakness of the supports themselves.  Completely inaccessible areas may require other repair alternatives, such as penetrating coatings or isolation and dehumidification of the entire house tank area in order to hopefully slow the corrosion process.  Lay-up chemicals that will effectively penetrate rust deposits and protect the underlying metal surfaces exist, and offer an excellent option in some applications.

• Threat To Other Services

While common to domestic water steel holding tanks, high exterior wall loss is possible for any vessel which operates at very low temperatures in the presence of humidity.  Such may exist at brine and ammonia refrigeration systems even though they are typically protected from moisture condensation through the use of heavy insulation.  A periodic visual inspection is therefore always advised.

Any breaks in the insulation will provide an entry point and high localized wall loss.  This is especially true at any threaded fixtures which are inherently difficult to properly insulate.  Small diameter pipe also provides a lower wall thickness, is reduced 50% or more due to threading losses – placing any inlet and out piping often at greater risk.  The application of a high quality protective coating is well recommended.

While there are many examples where corrosion cannot be controlled, the deterioration of any cold water tank due to lack of maintenance is generally avoidable.  We offer an extensive photos of different internal and corrosion issues related to domestic water and fire protection water storage tanks in our Tank Photo Gallery under our Photo Galleries.

© Copyright 2023 – William P. Duncan, CorrView International, LLC