UNIVERSITI KUALA LUMPUR

MALAYSIAN INSTITUTE OF MARINE ENGINEERING

TECHNOLOGY

SHIP MATERIALS
(LMB11503)

NAMA NO. ID
GROUP MAISARAH HANANI BINTI MOHD SHAH 56214218040
MEMBERS
MUHAMMAD ARIFFUDDIN BIN ZULKIFLE 56214217044
MUHAMMAD ARIF BIN DZKULFLI 56214218045

GROUP L01

TOPIC CORROSION IN BOILER

LECTURER DR ASMALINA BINTI MOHAMED SAAT

DATE 19/11/2018

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Contents Page
Introduction ............................................................................................................................................ 3
Introduction to Boiler ......................................................................................................................... 3
What is meant by corrosion in boilers? .............................................................................................. 5
PROBLEM AREAS OF BOILERS ................................................................................................................. 5
TYPES OF CORROSION............................................................................................................................. 8
Galvanic Corrosion .............................................................................................................................. 8
Caustic Corrosion ................................................................................................................................ 9
Acidic Corrosion .................................................................................................................................. 9
Hydrogen Embrittlement .................................................................................................................. 10
Oxygen Attack ................................................................................................................................... 10
PROBLEMS BY CORROSION ................................................................................................................... 11
Fatigue Cracking ................................................................................................................................ 11
Steam Side Burning ........................................................................................................................... 11
EFFECTS OF CORROSION IN BOILER ...................................................................................................... 12
CAUSES OF BOILER CORROSION ........................................................................................................... 12
PREVENTIVE MEASURES ....................................................................................................................... 12
CORROSION CONTROL .......................................................................................................................... 13
CONCLUSION......................................................................................................................................... 13
REFERENCES .......................................................................................................................................... 13

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Introduction
Corrosion is one of the main causes of reduced reliability in steam generating systems. It is
estimated that problems due to boiler system corrosion cost industry billions per year.
Corrosion is the reversion of a metal to its ore form. Iron, for example, reverts to iron oxide as
the result of corrosion. The process of corrosion, however is a complex electro chemical
reaction and it takes many forms. Corrosion may produce general attach over a large metal
surface or it may result in pinpoint penetration of metal. Corrosion is a relevant problem caused
by water in boilers. Corrosion can be of widely varying origin and nature due to the action of
dissolved oxygen, to the iron being directly attacked by the water. While basic corrosion in
boilers may be primarily due to reaction of the metal with oxygen, other factors such as stresses,
acid conditions, and specific chemical corrodent may have an important influence and produce
different forms of attack. It is necessary to consider the quantity of the various harmful
substances that can be allowed in the boiler water without risk of damage to the boiler.
Corrosion may occur in the feed-water system as a result of low pH water and the presence of
dissolved oxygen and carbon dioxide.

Introduction to Boiler
A boiler is an enclosed vessel that provides a means for combustion and transfers heat to water
until it becomes hot water or steam. The hot water or steam under pressure is then usable for
transferring the heat to a process. Water is useful and cheap medium for transferring heat to a
process. When water is boiled into steam its volume increases about 1,600 times, producing a
force that is almost as explosive as gunpowder. This causes the boiler to be extremely
dangerous equipment and should be treated carefully. Liquid when heated up to the gaseous
state this process is called evaporation. The heating surface is any part of the boiler; hot gases
of combustion are on one side and water on the other. Any part of the boiler metal that actually
contributes to making steam is heating surface. The amount of heating surface of a boiler is
expressed in square meters. The larger the heating surface a boiler has, the more efficient it
becomes.

The boiler system is made up of:

 Feed water system

 Steam system
 Fuel system

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The feed water system provides water to the boiler and regulates it automatically to meet the
steam demand. The water supplied to boiler that is converted to steam is called feed water. The
sources of feed water are:

1. Condensate or condensed steam returned from the processes

2. Makeup water which is the raw water which must come from outside the boiler room
and plant processes.

The steam system collects and controls the steam produced in the boiler. Steam is directed
through a piping system to the point of use. Throughout the system, steam pressure is regulated
using valves and checked with steam pressure gauges. The fuel system includes all equipment
used to provide fuel to generate the necessary heat. The equipment required in the fuel system
depend on the type of fuel used in the system.

Figure 1 Close feedwater system.

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What is meant by corrosion in boilers?
It is the deterioration of metals due to oxidation. The present of water in an acidic
condition provides the electrolyte required for corrosion action. Corrosion is a phenomenon in
which atmospheric oxygen in the air or water reacts with the metal to form oxides. Corrosion
in industrial boilers is a phenomenon in which the oxygen that is dissolved in the water inside
the boiler reacts with the metal of the boilers. Ferrous component of the metal reacts with the
atmospheric oxygen in the water to undergo oxidation. This oxidation results in the formation
of ferrous oxides. Ferrous oxides lead to the formation of cavities and holes in the metal. This
gradual formation of cavities and holes in the metal is termed as pitting. Pitting is the main host
for the corrosion which ultimately destructs metal and its properties.

PROBLEM AREAS OF BOILERS

Many corrosion problems occur in the hottest areas of the boiler-the water wall, screen, and
superheater tubes. Other common problem areas include deaerators, feedwater heaters, and
economizers. Most industrial boiler and feedwater systems are constructed of carbon steel.
Many have copper alloy and/or stainless steel feedwater heaters and condensers. Some have
stainless steel superheater elements. In order to minimize boiler system corrosion, an
understanding of the operational requirements for all critical system components is necessary.
Parts of boiler affected:

1. Deaerators

Deaerators are used to heat feedwater and reduce oxygen and other dissolved gases to
acceptable levels. Corrosion fatigue at or near welds is a major problem in deaerators. Most
corrosion fatigue cracking has been reported to be the result of mechanical factors, such as
manufacturing procedures, poor welds, and lack of stress-relieved welds. Effective corrosion
control requires regular monitoring of operation, minimization of stresses during start-up,
maintenance of stable temperature and pressure levels.

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 Figure 2 Deaerator

2. Feedwater Heaters
Boiler feedwater heaters are designed to improve boiler efficiency by extracting heat from
streams such as boiler water blowdown or excess exhaust steam. The primary problems occur
in feed water heaters are corrosion due to oxygen and improper pH, and erosion from the tube
side or the shell side. Due to the temperature increase across the heater, incoming metal oxides
are deposited in the heater and then released during changes in steam load. Corrosion can be
minimized through periodic cleaning, control of oxygen, proper pH control, and the use of
high-quality feedwater.

Figure 3 Feedwater heater

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 3. Economizers
Economizers help to improve boiler efficiency by extracting heat from flue gases discharged
from the fireside of a boiler. Economizers can be classified as nonsteaming or steaming. In a
steaming economizer, 5-20% of the incoming feedwater becomes steam. Steaming
economizers are particularly sensitive to deposition from feedwater contaminants and resultant
under-deposit corrosion. Erosion at tube bends is also a problem in steaming economizers.
Oxygen pitting, caused by the presence of oxygen and temperature increase, is a major problem
in economizers; therefore, it is necessary to maintain essentially oxygen-free water in these
units. Economizer heat transfer surfaces are subject to corrosion product build up and
deposition of incoming metal oxides.

Figure 4 Economizer

4. Superheater
Superheater corrosion problems are caused by a number of mechanical and chemical
conditions. One major problem is the oxidation of superheater metal due to high gas
temperatures, usually occurring during transition periods, such as start-up and shutdown.
Deposits due to carryover can contribute to the problem. Resulting failures usually occur
in the bottom loops-the hottest areas of the superheater tubes

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 Figure 5 Boiler (superheater location)

TYPES OF CORROSION
Galvanic Corrosion
Galvanic corrosion occurs when a metal or alloy is electrically coupled to a different metal or
alloy. The most common type of galvanic corrosion in a boiler system is caused by the contact
of dissimilar metals, such as iron and copper. These differential cells can also be formed when
deposits are present. Galvanic corrosion can occur at welds due to stresses in heat-affected
zones or the use of different alloys in the welds. Anything that results in a difference in
electrical potential at discrete surface locations can cause a galvanic reaction. Causes include:

a) Scratches in a metal surface.

b) Differential stresses in a metal.
c) Differences in temperature.
d) Conductive deposits.

Figure 6 Galvanic corrosion

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Caustic Corrosion
Concentration of caustic (NaOH) can occur either as a result of steam blanketing (which allows
salts to concentrate on boiler metal surfaces) or by localized boiling beneath porous deposits
on tube surfaces. Caustic corrosion (gouging) occurs when caustic is concentrated and
dissolves the protective magnetite (Fe3O4) layer. Iron, in contact with the boiler water, forms
magnetite and the protective layer is continuously restored. However, as long as a high caustic
concentration exists, the magnetite is constantly dissolved, causing a loss of base metal and
eventual failure. Caustic attack creates irregular patterns, often referred to as gouges.
Deposition may or may not be found in the affected area.

Figure 7 Caustic corrosion

Acidic Corrosion
Low feed water pH can cause serious acid attack on metal surfaces in the pre boiler and boiler
system. Even if the original makeup or feed water pH is not low, feed water can become acidic
from contamination of the system. Acid corrosion can also be caused by chemical cleaning
operations. Overheating of the cleaning solution can cause breakdown of the inhibitor used,
excessive exposure of metal to cleaning agent, and high cleaning agent concentration. Failure
to neutralize acid solvents completely before start-up has also caused problems

Figure 8 Acidic corrosion

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Hydrogen Embrittlement
Hydrogen embrittlement of mild steel boiler tubing occurs in high-pressure boilers when
atomic hydrogen forms at the boiler tube surface as a result of corrosion. Hydrogen permeates
the tube metal, where it can react with iron carbides to form methane gas, or with other
hydrogen atoms to form hydrogen gas. These gases evolve predominantly along grain
boundaries of the metal. The resulting increase in pressure leads to metal failure.

Figure 9 Hydrogen embrittlement

Oxygen Attack
Without proper mechanical and chemical daeration, oxygen in the feedwater will enter the
boiler. Much is flashed off with the steam; the remainder can attack boiler metal. The point of
attack varies with boiler design and feedwater distribution. Pitting is frequently visible in the
feedwater distribution holes, and at the steam drum waterline. Oxygen is highly corrosive when
present in hot water. Even small concentrations can cause serious problems. Because pits can
penetrate deep into the metal, oxygen corrosion can result in rapid failure of feedwater lines,
economizers, boiler tubes, and condensate lines. Additionally, iron oxide generated by the
corrosion can produce iron deposits in the boiler.

Figure 10 Oxygen attack

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PROBLEMS BY CORROSION
Fatigue Cracking
Fatigue cracking (due to repeated cyclic stress) can lead to metal failure. The metal failure
occurs at the point of the highest concentration of cyclic stress. Examples of this type of failure
include cracks in boiler components at support brackets or rolled in tubes when a boiler
undergoes thermal fatigue due to repeated start-ups and shutdowns. Thermal fatigue occurs in
horizontal tube runs as a result of steam blanketing and in water wall tubes due to frequent,
prolonged lower header blowdown.

Figure 11 Fatigue cracking

Steam Side Burning
Steam side burning is a chemical reaction between steam and the tube metal. It is caused by
excessive heat input or poor circulation, resulting in insufficient flow to cool the tubes. Under
such conditions, an insulating superheated steam film develops. Once the tube metal
temperature has reached 750°F in boiler tubes or 950-1000°F in superheater tubes (assuming
low alloy steel construction), the rate of oxidation increases dramatically; this oxidation occurs
repeatedly and consumes the base metal. The problem is most frequently encountered in
superheaters and in horizontal generating tubes heated from the top.

Figure 12 Steam side burning

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EFFECTS OF CORROSION IN BOILER
1. Pinpoint penetration of metal

2. Rusting of ferrous metals

3. Pits can penetrate deep into the metal that can result in rapid failure of feed lines,
economiser tubes and boiler tubes

4. Ultimate failure of boiler metal, steam mains and condensate lines

CAUSES OF BOILER CORROSION

 Improper and unscheduled maintenance.
 Exposure of boiler metal to dissolved gases present in boiler water
 Oxygen present in boiler water
 Carbon Dioxide present in boiler water
 Unbalanced pH of boiler water
 Improper and unscheduled maintenance
 Exposure of metal to the dissolved gases i.e oxygen and carbon dioxide present in
boilerfeed water
 Unbalanced pH of the water inside the Industrial Steam Boiler
 Improper and unscheduled maintenance

PREVENTIVE MEASURES
1. Eliminating corrosive gases
2. Removal of dissolved oxygen
3. High PH value of boiler water
4. Mechanical deaeration of boiler water
5. Higher feed water temperature i.e. reduces its oxygen content.
6. Chemical de-oxygenation by use of oxygen scavengers i.e. sodium sulphite
7. Hot condensate return as it contains less O2 then feed water and also saves fuel.

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CORROSION CONTROL
Many corrosion problems are the result of mechanical and operational problems. The following
practices help to minimize these corrosion problems:

 Selection of corrosion-resistant metals.

 Reduction of mechanical stress where possible (e.g., use of proper welding procedures
and stress-relieving welds).
 Minimization of thermal and mechanical stresses during operation.
 Operating within design load specifications, without over-firing, along with proper
start-up and shutdown procedures.
 Maintenance of clean systems, including the use of high-purity feedwater, effective and
closely controlled chemical treatment, and acid cleaning when required.
 Maintenance of proper pH throughout the boiler feedwater, boiler, and condensate
systems is essential for corrosion control.

CONCLUSION
Concluding, effective corrosion control monitoring is essential to ensure boiler reliability. A
well planned monitoring program should include the following:

 Proper sampling and monitoring at critical points in the system.

 Completely representative sampling.
 Use of correct test procedures.
 Checking of test results.
 A plan of action to be carried out promptly when test results are not within established
limits.
 An efficient plan for major upset conditions.
 A quality improvement system and assessment of results based on testing and
inspections.

REFERENCES
1. https://www.scribd.com/document/244637708/Boiler-Corrosion
2. http://www.machineryspaces.com/closed-feed-system.html
3. https://www.lenntech.com/applications/process/boiler/oxygen-attack.htm
4. https://en.wikipedia.org/wiki/Deaerator
5. https://www.brighthubengineering.com/marine-engines-machinery/74949-inspection-
carried-out-in-boiler-superheater-and-in-steam-drum/

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