Corrosion of Residential Above Ground Heating Oil Tanks

FEATURE

Corrosion of Residential Aboveground Heating Oil Tanks:An Overview

Marina Banuta • Isabelle Tarquini

Submitted: 28 September 2009 / in revised form: 18 December 2009 / Published online: 7 January 2010

� ASM International 2010

Abstract Three basic modes of failure are known for oil

tanks: manufacturing defects, mechanical damage, and

corrosion. Most of the tanks currently in use and manu-

factured in or prior to the 90s, are subject to leaking caused

by internal corrosion. Corrosion-induced leakage always

results in environmental damage by underground and/or

aboveground oil contamination. Considering the costs

related to decontamination and to replacement of residen-

tial tanks, the possibility of corrosion should be carefully

addressed. To accomplish this task, the most common

issues related to tank corrosion must be recognized and

understood. This article is an overview of the corrosion

mechanisms in aboveground residential tanks made of non-

coated mild steel.

Keywords Pitting corrosion � Oil tanks � Water �Heating oil

Introduction

Heating oil tanks are very popular components of resi-

dential heating systems in North-America. Their life

service usually ranges between 10 and 15 years but can be

significantly longer. However, in some cases premature

shell perforation occurred after only one to two years of

service [1]. The great majority of the residential tank leaks

evaluated becomes the object of failure analysis because of

the need to assess the responsibility for the spilling. Even

though heating oil tank leakages are seldom a spectacular

event, spilling from residential tanks may cause material

and environmental damage and can affect lives and homes.

This article will try to provide insight into the degradation

phenomena and explain why tanks fail and what to do to

minimize the unwanted leakage. Unfortunately, the failure

analyst is (almost) always called late to failures that have

potential litigations, thus, it is very difficult to achieve the

goal of prevention. However, the authors sincerely hope

that after reading this article, some one will take time to

spread the message and advice relatives, friends, or

neighbors who possess one of these tanks and begin the use

of ‘‘backyard prevention technologies.’’

Figure 1 shows a sketch of an aboveground heating oil

residential tank, manufactured according to Canadian

standard CAN/ULC-S602: Aboveground Steel Tanks for

Fuel Oil and Lubricating Oil. While the latest version of

this standard was released in 2003, it must be noted that the

great majority of the tanks still in use were made in

accordance to the 1992 version, thus demonstrating that

tank lifetimes generally exceed 10 years. The most com-

mon residential tank has a maximum nominal capacity of

1200 l, and a minimum shell thickness of about 2 mm

(0.078 in.). The tank is made of welding quality mild steel,

in accordance with the latest edition of ASTM A569 and/or

CAN3-G40.21M 230G or a recognized equivalent. Usually

having an obround shape, a residential tank is composed of

a metallic shell (that may be fabricated from two or three

pieces of steel sheet) welded to two heads (that must be

fabricated from not more than one piece of steel). All shell-

and head-welded joints must be in accordance with the

specifications of the above-mentioned standard. A leak test

is performed on every tank, at the end of the manufacturing

process.

The dimensions and locations of the fill, vent, and gauge

are stated in the standard and are shown in Fig. 1. The

M. Banuta (&) � I. Tarquini

SGS Canada, Materials Engineering, Montreal, QC, Canada

e-mail: [email protected]

123

J Fail. Anal. and Preven. (2010) 10:69–76

DOI 10.1007/s11668-009-9322-2

burner supply may be located: (1) below the liquid level

line, (2) in the tank’s head, or (3) in the tank’s bottom. As

will be shown later, the burner supply’s location was

identified as a critical element in the tank service life and

corrosion resistance. Most manufacturers have chosen to

place the burner supply in the tank bottom. As per the

standard specification, every tank shall be equipped with no

less than four support brackets or two cradles of substantial

design and construction. Regardless of the method, the tank

must be rigidly supported. Even though the standard makes

no reference to installation issues, all installation guides

recommend a slope of 1/400/ft of tank length, with the

burner supply end being lower, in order to facilitate

drainage and oil flow.

Finally, it must be noted that in old tanks the internal

surfaces were bare steel, without any form of corrosion

protection. External surface are usually cleaned and

painted.

Typical Degradation Mode and Causes of Corrosion

in Aboveground Heating Oil Tanks

Three basic modes of failure are known for oil tanks:

manufacturing defects, mechanical damage, and corrosion.

However, the most prevalent cause of failure in above-

ground residential heating oil tanks is internal corrosion

which ultimately leads to the perforation of the shell. The

corrosion mechanism is pitting corrosion, which is a very

insidious process as it causes little material loss and may

not be noticed by visual examination of the metal surface

even when the pit has penetrated deep within the material

and the shell structure is damaged, almost to the point of

leakage [2]. Corrosion products often obscure the pits

formation, thus making almost impractical to discover the

shell perforation before leakage. In most cases, internal

corrosion occurs at the tank bottom, and depending on the

installation conditions (indoor or outdoor), external corro-

sion may also occur.

As a form of localized corrosion, pitting corrosion is

generally associated with the presence of a stagnant liquid

on the metallic surfaces. The stagnant liquid usually at a

tank bottom and is a mix of heating oil, sludge, and water.

Even though not readily identifiable, water is almost

always present and plays a significant role in aboveground

tanks corrosion because heating oil and sludge are poor

electrolytes. In our experience with residential above-

ground heating oil tanks, the shell perforation always

occurred at the bottom, in an area containing a ‘‘water

strip’’, which is considered positive proof that water and

sludge were present at the tank bottom some time prior the

leaking. It is important to note that, especially in old tanks,

the design of the tank bottom and the location of the burner

opening are particularly prone to accumulations of water

and sludge. In fact, recent failure analyses of these tanks

pinpointed design as the critical issue in the tanks’ service

life. Because of this observation, the newer designs place

the burner opening directly on the bottom and the accu-

mulations became less frequent.

While the heating oil and sludge mix at the tank bottom

are related to the tank content, water can be considered an

intruder. First, a small quantity of water can be contained in

the heating oil. The maximum water content allowed by the

Canadian standards is 0.05% [3]. When this requirement is

met, water is usually not harmful. However, there are many

ways supplementary water can enter an oil tank. Transfer

of water and sludge from an old tank, which is strictly

prohibited by the manufacturer’s, is one of the most

common way. Indeed, it is considered that new steel tanks

are initially more susceptible to corrosion caused by the

presence of sludge, acids, microorganisms, and water than

are old tanks. Evidently, tank service improves the passive

qualities of the surface film. Transferring oil from an old

tank (the customer wants to use the oil that was left) also

places contaminants in a new steel tank and can result in

premature failure. As an example, the authors were con-

sulted in a case of multiple premature failures which could

be attributed to transfer of the contents of old tanks into

new ones. The service life of the new tanks was under

18 months.

In outdoor tanks, moisture condensation phenomena is

another important source of water, often interrelated to

weather condition, temperature variations, and the location

of the tank. The quantity of water obtained by condensation

may be significant in summer, when most tanks are not

filled to the allowed capacity. In indoor tanks, condensation

Fig. 1 Schematic representation of a heating oil residential tank

70 J Fail. Anal. and Preven. (2010) 10:69–76

123

usually occurs when the brand new tank is installed

because new tanks could experience internal condensation

due to temperature variations during transportation or

storage. Finally, additional water can be introduced directly

from the oil truck or through a defective or missing vent

and/or fill cap.

Since water is heavier than oil, it will go to the tank

bottom. The oil above will not allow water evaporation and

as such, water combined with aggressive elements con-

tained by the heating oil, will stay and promote pitting

corrosion. Hydrogen sulfide (H2S) and chlorine salts

(NaCl, KCl, CaCl2) are commonly contained in heating oil

and represent corrosive species for mild steel. Also, sulfur

coming from oil combines with water to form a highly

corrosive species for the tank shell, sulfuric acid (H2SO4).

Some sources indicate that the sulfuric acid aggressiveness

toward the carbon steel is increased in the presence of

chlorine salts [4, 5].

Microorganisms are another concern in the corrosion of

heating oil tanks, especially microbes producing H2S and

H2SO4 as by-products of their metabolic activities [5].

These acids produced by the microbes will add to the

aggressiveness of the environment and the biofilms created

by the microorganisms at the tank bottom may enable the

formation of strong electrochemical cells which promote a

very localized corrosion and pitting. In our experience,

shell perforations occurred more often than not at the site

of microorganisms’ colonies.

Case Study

The authors of this article are often called to give their

opinion in cases involving residential oil tanks, as part of

claim adjusting procedures. Even though the analyzed

tanks came from different manufacturing companies and

from different Canadian regions, they always exhibited the

corrosion issues described above. The following example

will illustrate the typical damage observed in these tanks.

Background

The residential tank presented in this article failed during

winter 2008, in a residence near Montreal, Canada. The

tank was installed indoors, but no information concerning

its installation was provided. At the time of leakage the

tank was 13 years old, having been fabricated in 1995.

According to the manufacturing label attached to the shell,

it had 1135 l capacity and the metal thickness was 2 mm.

After the incident, the tank was emptied by the owner and

kept outdoors for about a month, after which was trans-

ferred in a warehouse. As such, when our team was called

to investigate the cause of failure, the tank was no longer at

the incident site and none of the connections were present.

Our investigation was conducted in cooperation with

experts representing the other interested parties. This case

study will present a summary of the results of the failure

analysis performed by the authors.

General External Examination

As our measurements indicated, the general dimensions

and the location of gauges, vents, and fills of the failed tank

met the ULC standard requirements (1992 release). The

burner supply was located on one of the heads, at

approximately 70 mm height, once again according to the

applicable standard. No evidence of mechanical damage

was noted.

General external examination showed that the failed

tank was a painted obround domestic fuel tank. The metal

label found on the tank indicates that the tank was fabri-

cated in 1995, by a Canadian manufacturer, according to

standard CAN/ULC-602-M92: ‘‘Standard for Aboveground

Steel Tanks for Fuel and Lubricating Oil.’’ The tank

apparently experienced perforation at the bottom line,

located towards the burner opening side (Figs. 2, 3). In this

area, four round indications with diameters ranging from 1

to 4 mm were noted at the bottom line, in the vicinity of the

Fig. 2 General view of the tank bottom line

J Fail. Anal. and Preven. (2010) 10:69–76 71

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burner opening (area shown in yellow in Fig. 2). Visually,

only two of them seem to have penetrated the tank’s shell,

being responsible for the leakage, while the others showed

no visible evidence of penetration (Fig. 3).

At several locations, the paint seems to have been

slightly damaged by physical contact with another surface

(Figs. 2, 3), although it was difficult to assess the exact

moment of the damage. Close-up examination of the area

containing the discontinuities found the presence of several

spatters (Fig. 4). They were also noted at another locations

on the tank shell, near welded beads. Spatters are small

metal particles, which are thrown out of the arc during

welding and get deposited on the regions around the weld

bead. Although weld spatter is not exactly a weld defect, it

is usually recommended to be avoided and, in case it

happens, it should be cleaned and removed [6].

General Internal Examination

The tank was sampled and cut open with a nibbler machine

to allow full internal visual examination. Visual inspection

of the internal surfaces of the tank revealed the existence of

a corroded strip area over the tank bottom line, fully cov-

ered with thick moist deposits (Fig. 5a, b). The corroded

strip, usually associated with the presence of stagnant

liquid at the tank bottom, has a variable width, being wider

toward the burner side, its size and shape indicating the

tank was inclined toward the burner side (Fig. 5a). As

already explained, this is current practice among the tank

installers companies and its objective is to facilitate the oil

flow to the burner and better drainage.

Fig. 3 Location and size of the indications noted in the tank’s

damaged area

Fig. 4 Presence of spatters associated with the adjacent welded beads

Fig. 5 (a) Image of the corrosion stripe toward the burner side.

(b) Image of the corrosion stripe opposite the burner side


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It was also noted that some of the tank content, mixed

with sludge and possibly water, was still present at the

investigation time. The said tank content, as well as the

moist deposit present at the tank bottom line, were sampled

for further analysis. Before starting the sampling of the

damaged area, the surface was cleaned with an industrial

solvent to allow visual examination of the corroded strip at

the tank bottom line. Thus, it was noted that under the

deposit, pitting corrosion was already installed and, at

certain locations, the damage in the tank shell seems rel-

atively profound (Fig. 6). No evidence of corrosion was

noted at locations other than those related to the strip at the

tank bottom.

The section of the tank bottom containing all the iden-

tified discontinuities was cut and divided in four

specimens, one for each party. One sample, consisting of a

transverse section of the tank bottom shell, near the burner

opening side, was transported to our facility for further

analysis (Fig. 7). During the sampling, it was noted that

some of the discontinuities apparently showing no pene-

tration through the tank shell, were in fact clogged with

corrosion products and deposits. They were easily cleared

from shell using a metallic pin.

General Examination of the Tank Sample

General examination was conducted on both external and

internal sides of the tank shell section. Visual examination

of the external tank surface revealed the presence of several

spatters, most of them being covered with paint. Few of

them exhibit shiny metallic surfaces indicating a grinding

effect at the contact with another surface (Fig. 8). The

presence of the previously identified indication # 1 was

confirmed and the existence of a second discontinuity

was noted. The latter became visible only during

low-magnification examination. Although apparently not

completely penetrating the shell, this indication turned into

a hole when low-pressure air was used to clean the adjacent

surface. It was noted that the hole has a perfectly round

shape at the external surface of the shell, which reminds of

the shape of adjacent spatters and may indicate that a

spatter could be at its origin. However, no physical evi-

dence indicates that the corrosion process originated

outside the tank. It was thought that the presence of the

spatter may rather have accelerated the penetration from

the inside.

Fig. 6 Evidence of pitting corrosion in the corrosion stripe, after

sampling and cleaning the residues

Fig. 7 Bottom tank section, near the burner side

Fig. 8 Close-up view of the external surface of the tank shell, in the

damaged area


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Low-magnification and visual examination were next

conducted on the internal surface of the tank shell. Closer

view of the affected area showed evidence of corrosion

pits, typically associated with pitting corrosion mecha-

nisms (Fig. 9). Although pitting corrosion in progress is

present over the area formerly covered with moist deposits,

intense corrosion damage is present at the internal surface

of the shell at the location of the two perforations noted

from the outside, which indicates that the perforation

occurred from the inside to outside the tank shell.

Materials

The chemical composition of the tank shell was determined

by spectroscopy chemical analysis. The results, shown in

Table 1, are in accord with the requirements of the

Canadian standard CSA G40.21-M92, for grade 230 G.

Microscopic Examination

Microscopic examination was conducted in the corroded

area of the tank sample, in the area of indication # 1, in

order to validate the corrosion mechanism and to evaluate

the damage magnitude in the shell cross section. The

metallographic cut was performed as to obtain two trans-

verse metallographic specimens, both of which were

Bakelite mounted and mirror polished. Microetching using

Nital 3% reagent was performed in order to reveal the

details of the microstructure. Figures 10 and 11 show

general images of the analyzed surfaces of the metallo-

graphic specimens.

According to the observations conducted on these

specimens, the tank perforation has indeed been produced

by pitting corrosion, from inside the tank shell to outside.

The original thickness of the shell measured in undamaged

areas was found to be of 2 mm. On the observed speci-

mens, thickness loss of about 50% was revealed in the

shallow pitting area (area not yet penetrated), which indi-

cates in-progress corrosion process.

High-magnification examination in the degraded area

showed that the corrosion progressed in both transverse and

longitudinal directions, according to a typical pitting

corrosion phenomenon (Figs. 12, 13). Finally, the tank

microstructure was found to be composed of ferrite and

Fig. 9 Close-up view of a perforation, looking from inside out

Table 1 Chemical composition of the tank shell

Elements Tank shell, wt% CSA G40.21-M92 Grade 230 G, wt%

C 0.07 0.26 max.

Mn 0.27 1.20 max.

P 0.012 0.050 max.

S 0.004 0.050 max.

Si 0.01 0.40 max.

Fig. 10 General view of the perforation (cross section)

Fig. 11 General view of the shell in a corroded but not penetrated

area


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network-like pearlite, typically found in normalized struc-

tures of low-carbon steels (Fig. 13).

Sludge and Deposits Analysis

The sludge sampled on the tank bottom during the cutting,

was analyzed in order to find evidence of corrosive species.

The obtained results are presented in Table 2.

There is no doubt these chemicals came from the heat-

ing oil and from the sludge at the tank bottom. They may

have been contained in the heating oil, along with a mix-

ture of hydrocarbons. Indeed, literature mentions the

presence of some salts, like sodium chlorine (NaCl),

calcium chlorine (CaCl), potassium chlorine (KCl), and

magnesium chlorine (MgCl), in the heating oils [5].

Hydrogen sulfide (H2S) and microorganisms are equally

present [5]. Sulfur and chlorine are considered as corrosive

species for the mild steel, especially when combined with

water. Moreover, it is thought that the presence of chlorine

salts, like those indicated above, accelerates the corrosion

rate.

Conclusion

Considering the typical corrosion damage experienced by

the residential heating oil tanks, as found in all cases

investigated by the authors, it can be noted that the main

cause of failure is the presence of water. Unlike the other

chemicals inherently present in the heating oil, the water

is an intruder. However, most of the sources of water in

residential tanks can be ‘‘user-controlled’’. As such, it is

obvious that owner education could have a significant

impact on the tank service life. From our experience, the

new tanks currently being sold always have enough

documentation to explain to the owners how to properly

install and use their tanks so as to avoid premature

corrosion failure. Literature is also available on the

internet. However, it is common knowledge that very

few of the owners read installation manuals. Unfortu-

nately, since as we come (almost) always late in this

kind of incidents, it is very difficult for us to achieve our

goal in prevention.

For that reason, the authors sincerely hope that after

reading this article, some of us will take time to spread the

message and to advice relatives, friends, and neighbors

that:

• If you buy a non-coated tank, avoid those models that

have the burner supply connection high in the head.

• Never allow full transfer of the content of a old tank in

a new one. Such transfer is one of the best ways to

produce premature corrosion failures in your newly

acquired tank.

• Ensure no condensation water is present in the tank

when you buy it.

• Install the tank as to allow drainage.

• If you keep your tank outdoors, choose a location where

the temperature gradients are minimum. This will

prevent condensation inside your tank and water

accumulation.

• Keep your tank always at the recommended capacity,

even in summer, in order to diminish the quantity of

condensation water.

Fig. 12 Typical micrograph in the perforation wall (2009)

Fig. 13 Close-up view of a shallow pit (8009)

Table 2 Sludge chemical analysis

Elements Sludge sample, ppm

Ca 1053

Na 776

Cl 95

K 144

S total 2150


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References

1. Banuta, M.: Revue d’un rapport d’etude de corrosion sur un

reservoir de mazout non enterre (Review of a corrosion study of an

aboveground heating oil tank). Company Investigation Report

(2009) (in French)

2. Corrosion: Metal Handbook, vol. 13, 9th edn, p. 113. ASTM

International, Materials Park, OH (1998)

3. CAN/CGSB3.2-1999: Heating Fuel Oil. National Standard of

Canada (2007)

4. Corrosion: Metal Handbook, vol. 13, 9th edn, p. 40. ASTM

International, Materials Park, OH (1998)

5. Groysman, A.: Corrosion of aboveground fuel storage tanks.

Mater. Perform. 44(9), 44 (2005)

6. CAN/ULC-S602-M92: Standard for Aboveground Steel Tanks for

Fuel Oil and Lubricating Oil, National Standard of Canada and

Underwriters’ Laboratories of Canada, art. 4.1, 3rd edn., 1992


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Corrosion of Residential Above Ground Heating Oil Tanks