Preventing corrosion at the base of carbon steel tanks: a few simple steps to prevent external corrosion of metal tanks can help prevent a range of consequences, from minor leaks to catastrophic spills.

External corrosion of metal tanks in pulp and paper mills is a widespread problem. Two forms of external corrosion are most common: thinning of the wall and floor around the base of carbon steel tanks and at insulation or stiffener rings, and chloride stress corrosion cracking of stainless steel tanks. The presence of insulation accentuates these problems, as discussed below.

The results of external corrosion can range from inconsequential cracking or thinning, to minor leaks, to catastrophic rupture or spillage of the tank contents, which can threaten people and the environment. Hot contents pose greater risks and aggressive chemical contents such as sodium chlorate and sulfuric acid can make even small spills or leaks especially troublesome.

Predictably, more corrosion occurs when the "time of wetness" is longer. The nature of the wetness plays a role too. Alkaline spills and washup environments are less corrosive than neutral environments, whereas acidic environments can cause rapid attack. Common sources of the latter are acid spills and decaying chips or sawdust around tank bases. Rain, washups, seal water for agitators, and steam leaks are the most prolific sources of water.

The practice of installing external insulation all the way down to the base plate at the bottom of tanks increases the likelihood and severity of corrosion there. Most ordinary, sheet-lagged insulation materials collect and hold wetness against the tank wall at the bottom and at non-draining insulation rings at other elevations on the tank. Corrosion can also occur where chips, bark, or other soakable materials accumulate around the tank or on its roof.

The nature of the insulation can influence the rate of corrosion. Fibrous materials and rock wool products tend to hold wetness longer than rigid Foamglass[R] or other, less porous, insulation materials; however, all materials absorb and hold some moisture. Use of rigid materials does not prevent corrosion.

Insulation or debris not only creates a harmful, wet "compress," it also hides the corroding surfaces. This can produce unpleasant surprises when the corroding surface is eventually exposed or, more spectacularly, when the tank empties through a leak or bursts. Typical symptoms of this problem are shown schematically in Figure 1.

[FIGURE 1 OMITTED]

The condition of the base plate edge projecting out around the bottom of the tank oftens serves as an important "indicator" of the severity of the corrosion. If this lip has disappeared or is severely corroded--it often develops an "alligatored" appearance due to end-grain attack--both the tank wall above it and the underside of the tank floor should be inspected without delay.

PREVENTING TANK BASE CORROSION

Preventing external corrosion of metal tanks is relatively simple, and can be done at minimal cost. The basis for preventing the corrosion is to eliminate the harmful "wet compress" effect of insulation (or debris) wherever it may occur on the tank, but especially at the tank base.

Insulation systems are practically impossible to keep leak-tight. Therefore, corrosion around the base of a carbon steel tank can best be prevented by removing the insulation from the bottom foot or so of the tank and applying a suitable protective coating to the bared metal. This achieves two critical results: it eliminates the "wet compress" effect of insulation and it makes it simple to inspect the base of the tank, thereby eliminating unwanted surprises.

The protective coating should preferably be a self-priming, high build, immersion-grade coating resistant to wet environments. Coal tar epoxy, cycloaliphatic amine epoxy and novolac epoxy materials typically give good performance; they are listed here in order of increasing cost and increasing general chemical resistance.

All coatings depend on good surface preparation to provide the desired performance. All the coating types recommended above require thorough surface decontamination and sandblasting to at least SSPC SP6, or "commercial," cleanliness (although SSPC SP10, "near white," is better.) Some coatings can be applied in only one coat. Widespread use of coal tar epoxy paint, which normally is black, prompts some to call tanks protected in this way "booted," because the tank looks like it is wearing black boots.

Many mills find that a course of rigid, Foamglass[R] insulation blocks helps stabilize and close off the bottom of the shortened insulation jacket. In addition, mills should eliminate conditions that retain water around the tank baseplate, or that redirect water underneath the tank. This may involve grinding drainage channels in or reshaping the tank's ring wall or foundation and inserting a durable seal (a compressed rubber tube is recommended) around the edge of the tank base to keep water or other liquids from getting beneath the tank floor.

Preventing replenishment of wetness under the tank by sealing new wetness out is more important than worrying about any moisture trapped under the tank by such a seal. In most cases the moisture under the tank must have air in it to be corrosive; a good seal reduces access by air under the tank. (This explains why under-floor corrosion on carbon steel tanks normally is most severe in a 2- or 3-foot-wide band around the tank periphery.)

Figure 2 shows schematically how to apply the recommended preventive measures on both new and repaired tanks. Corroded tank walls often can be repaired by adding properly designed reinforcing around the bottom of the wall, especially if the corrosion problem is addressed before the tank is excessively damaged.

[FIGURE 2 OMITTED]

David C. Bennett is principal engineer, materials and corrosion engineering, at M&M Engineering, Ramsey, New Jersey, USA. Bennett is co-instructor for the award-winning TAPPI Short Course on Solving Corrosion Problems in Pulp and Paper Industry. He can be reached at [phone], or by email at [e-mail]