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CHAPTER 5

CORROSION STUDIES

5.1 INTRODUCTION

This chapter deals with the corrosion mechanisms taking place in

aluminum and ceramics, the materials of interest in this study, the various

surface treatments adopted to reduce the effects of corrosion and the corrosion

tests conducted on mullite coated cast aluminum. Through literature survey it

was found that mullite is a ceramic material with good corrosion resistance

against acids and alkalies. Literature reports confirm the use of mullite coated

SiC substrates for harsh corrosive environments. Mullite has also been

applied on Yttria stabilized zirconia (YSZ) coated specimens to protect the

YSZ layer against corrosion, due its better corrosion resistance. An attempt

has been made in this thesis work to study the corrosion resistance of the

duplex coated mullite cast aluminum specimens using accelerated corrosion

tests conducted at room temperature. Research material in this area was found

to be scanty. The corrosion resistance of the duplex coating under service

conditions, as in an IC engine, has been studied by endurance road test of the

engine with the coated piston and cylinder head. The coated surfaces have been

observed for any signs of corrosion after the test and the result presented.

5.2 DEFINITION AND EXPLANATION OF CORROSION

Corrosion is the disintegration of an engineered material into its

constituent atoms due to chemical reactions with its surroundings (Fontana 2005).

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This means electrochemical oxidation of metals in reaction with an oxidant

such as oxygen. Formation of an oxide of iron due to oxidation of the iron

atoms in solid solution is a well-known example of electrochemical corrosion,

commonly known as rusting. This type of damage typically produces oxide(s)

and/or salt(s) of the original metal. Corrosion can also refer to other materials

than metals, such as ceramics or polymers. Many structural alloys corrode

merely from exposure to moisture in the air. Corrosion can be concentrated

locally to form a pit or crack, or it can extend across a wide area more or less

uniformly corroding the surface. Corrosion is a diffusion controlled process,

and it occurs on exposed surfaces. As a result, methods to reduce the activity

of the exposed surface, such as passivation and chromate-conversion, can

increase a material's corrosion resistance. However, some corrosion

mechanisms are less visible and less predictable.

5.3 TYPES OF CORROSION

A brief description of the various types of corrosion more relevant

to this study are given below (Fontana 2005).

5.3.1 Galvanic Corrosion

Galvanic corrosion occurs when two different metals electrically

contact each other and are immersed in an electrolyte. In order for galvanic

corrosion to occur, an electrically conductive path and an ionically conductive

path are necessary. This effects a galvanic couple where the more active metal

corrodes at an accelerated rate and the more noble metal corrodes at a retarded

rate. When immersed, neither metal would normally corrode as quickly

without the electrically conductive connection (usually via a wire or direct

contact). Galvanic corrosion is often utilized in sacrificial anodes like zinc

used for steel structures. The surface area ratio of the anode and cathode will

directly affect the corrosion rates of the materials. Given the right conditions,

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a thin film of corrosion products can form on a metal's surface spontaneously,

acting as a barrier to further oxidation. When this layer stops growing at less

than a micrometer thick, the phenomenon is known as passivation (rust, for

example, usually grows to be much thicker, and so is not considered

passivation, because this mixed oxidized layer is not protective). Passivation

in air and water at moderate pH is seen in such materials as aluminum,

stainless steel, titanium, and silicon.

5.3.2 Pitting Corrosion

Certain conditions, such as low concentrations of oxygen or high

concentrations of species such as chloride which compete as anions, can

interfere with a given alloy's ability to re-form a passivating film. In the worst

case, almost all of the surface will remain protected, but tiny local fluctuations

will degrade the oxide film in a few critical points. Corrosion at these points

will be greatly amplified, and can cause corrosion pits of several types,

depending upon conditions. Pitting remains among the most common and

damaging forms of corrosion in passivated alloys, but it can be prevented by

control of the alloy's environment.

5.3.3 Crevice Corrosion

Crevice corrosion is a localized form of corrosion occurring in

spaces to which the access of the working fluid from the environment is

limited and a concentration cell, areas with different oxygen concentration,

will take place with consequent high corrosion rate. These spaces are

generally called crevices. Examples of crevices are gaps and contact areas

between parts, under gaskets or seals, inside cracks and seams, spaces filled

with deposits and under sludge piles.

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5.3.4 High Temperature Corrosion

High temperature corrosion is chemical deterioration of a material

(typically a metal) under very high temperature conditions. This non-galvanic

form of corrosion can occur when a metal is subject to a high temperature

atmosphere containing oxygen, sulfur or other compounds capable of

oxidizing (or assisting the oxidation of) the material concerned. For example,

materials used in aerospace, power generation and even in car engines have to

resist sustained periods at high temperature in which they may be exposed to

an atmosphere containing potentially high corrosive products of combustion.

5.3.5 Corrosion in Ceramics

Most ceramic materials are almost entirely immune to corrosion.

The strong ionic and/or covalent bonds that hold them together leave very

little free chemical energy in the structure; they can be thought of as already

corroded. When corrosion does occur, it is almost always a simple dissolution

of the material or chemical reaction, rather than an electrochemical process. A

common example of corrosion protection in ceramics is the lime added to

soda-lime glass to reduce its solubility in water.

5.4 METHODS OF PROTECTION FROM CORROSION

5.4.1 Applied Coatings

Plating, painting, and the application of enamel are the most

common anti-corrosion treatments. They work by providing a barrier of

corrosion-resistant material between the damaging environment and the (often

cheaper, tougher, and/or easier-to-process) structural material. Platings

usually fail only in small sections, and if the plating is more noble than the

substrate (for example, chromium on steel), a galvanic couple will cause any

exposed area to corrode much more rapidly than an unplated surface would.

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For this reason, it is often wise to plate with a more active metal such as zinc

or cadmium.

5.4.2 Anodization

Anodizing is an electrochemical process that converts the metal

surface into a decorative, durable, corrosion-resistant, anodic oxide finish.

Aluminum is ideally suited to anodizing. Anodizing is accomplished by

immersing the aluminum into an acid electrolyte bath and passing an electric

current through the medium. A cathode is mounted to the inside of the

anodizing tank; the aluminum acts as an anode, so that oxygen ions are

released from the electrolyte to combine with the aluminum atoms at the

surface of the part being anodized. The anodic oxide structure originates from

the aluminum substrate and is composed entirely of aluminum oxide. This

aluminum oxide is not applied to the surface like paint or plating, but is fully

integrated with the underlying aluminum substrate, so cannot chip or peel. It

has a highly ordered, porous structure that allows for secondary processes

such as coloring and sealing.

Aluminum is actually a very active metal, meaning that its nature is

to oxidize very quickly. While a weakness for most metals, this quality is

actually the key to its ability to resist corrosion. When oxygen is present (in

the air, soil, or water), aluminum instantly reacts to form aluminum oxide.

This aluminum oxide layer is chemically bound to the surface, and it seals the

core aluminum from any further reaction. This is different from oxidation

(corrosion), in steel, where rust puffs up and flakes off, constantly exposing

new metal to corrosion. Aluminum’s oxide film is tenacious, hard, and

instantly self-renewing.

Although aluminum has a huge advantage when compared to other

metals, it is not always completely impervious to corrosion. Its protective

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oxide layer can become unstable when exposed to extreme pH levels. When

the environment is highly acidic or basic, breakdown of the protective layer

can occur, and its automatic renewal may not be fast enough to prevent

corrosion.

5.4.3 Thermal Spray Coatings for Corrosion Protection

Thermal spray coatings are widely used in preventing corrosion of

many materials, with very often; additional benefits of properties such as wear

resistance. Broadly, thermal spray coatings fall into three main groups:

Anodic Coatings, Cathodic Coatings and Neutral Coatings. Neutral coatings

use ceramics as coating materials to provide corrosion protection of metals

and discussed below.

Neutral materials such as alumina or chromium oxide ceramics

provide excellent corrosion resistance to most corrosive environments by

exclusion of the environment from the substrate. Generally a neutral material

will not accelerate the corrosion of the substrate even if the coating is

somewhat permeable and an exception to this is with stainless steel type

materials where the exclusion of oxygen can cause crevice corrosion, nickel

chromium bond coats are required to stop this and the densest and thickest

plasma sprayed coatings are recommended.

5.5 EXPERIMENTAL WORK

The corrosion resistance of the mullite coatings was tested using

the salt spray apparatus as per IS: 9000-1983 (Part XI) and the salt bridge

apparatus as per IS 15298 and the number of hours the specimens withstood

before the first sign of corrosion was noted.

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5.5.1 Salt Spray Test

It is a standardized test method used to check corrosion resistance

of coated samples. Coatings provide corrosion resistance to metallic parts

made of steel, aluminum or brass. Since coatings can provide a high corrosion

resistance through the intended life of the part in use, it is necessary to check

corrosion resistance by other means. Salt spray test is an accelerated corrosion

test that produces a corrosive attack to the coated samples in order to predict

its suitability in use as a protective finish. The appearance of corrosion

products (oxides) is evaluated after a period of time. Test duration depends on

the corrosion resistance of the coating; the more corrosion resistant the

coating is, the longer the period in testing without showing signs of corrosion.

Salt spray testing is popular because it is cheap, quick, well

standardized and reasonably repeatable. There is, however, only a weak

correlation between the duration in salt spray test and the expected life of a

coating, since corrosion is a very complicated process and can be influenced

by many external factors. Nevertheless, salt spray test is widely used in the

industrial sector for the evaluation of corrosion resistance of finished surfaces

or parts.

The testing equipment consists of a closed testing chamber, where a

salted solution (mainly, a solution of sodium chloride) is sprayed by means of

a nozzle. This produces a corroding environment in the chamber and thus,

parts in it are attacked under this severe corroding atmosphere. Typical

volumes of these chambers are of 420 litres. ISO recommends that the

chamber shouldn't be smaller than 200 litres by volume, in order to receive an

acceptable amount of test samples. Tests performed with a solution of NaCl

are known as NSS (neutral salt spray). Results are represented generally as

testing hours in NSS without appearance of corrosion products (e.g. 720 h in

NSS acc. to ISO 9227). Other solutions are acetic acid (ASS test) and acetic

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acid with copper chloride (CASS test), each one chosen for the evaluation of

decorative coatings, such as electroplated copper-nickel-chromium, electroplated

copper-nickel or anodized aluminum.

5.5.2 Salt Bridge Apparatus

Also, IS 15298 (Part 1): 2002 (ISO 8782-1: 1998), IS: 9000-1983

(Part XI), were used as standards for the study. The IS 15298 (Part 1)

procedure is given below. Figure 5.1 shows the salt bridge apparatus.

1. Filter paper 2. Specimen 3. Glass 4. Nacl solution (3%)

Figure 5.1 Salt bridge corrosion test

Remove any coatings of grease which may be present. Use a

1% (m/m) aqueous solution of sodium chloride as the test solution. Pour

approximately 200 mL of this solution into a porcelain dish and cover with a

sheet of glass leaving a small opening. Dip two strips of white filter paper of

dimensions at least 100 mm wide and 150 mm long into the test solution at

one end so that the strips of filter paper become saturated with solution, the

other ends being laid on the sheet of glass.

Lay the sample to be tested on one filter paper and cover the sample

with the other. Ensure the filter paper remains saturated throughout the test.

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After 48 hours remove the filter paper and examine the specimen for signs of

corrosion caused by the sodium chloride solution. Measure the size of each

area of corrosion in square millimeters and note the number of such areas.

The photographs of the tests conducted are shown in Figures 5.2 and 5.3.

Figure 5.2 Corrosion test in progress

Figure 5.3 Corrosion results after 500 hours of testing

5.5.3 Corrosion Test Under Service Conditions

The duplex mullite coated piston and cylinder head were assembled

in a petrol engine and an endurance test was conducted. The engine was made to

run on road for a duration of 1000 hours and then visually examined for any signs

of corrosion. The examination revealed no sign of corrosion products.

Specimen

Salt solution

Specimen

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5.6 RESULTS AND DISCUSSION

Three specimens of mullite coated specimens with bond coat of

nickel chrome were subjected to corrosion tests in a salt spray chamber with

5 % NaCl solution with specific gravity 1.0268 to 1.0413 and pH of 6.5 to

7.2. The specimens were tested for 48 hours and no signs of corrosion were

evident in all the samples. The test was called off after this duration. Another

set of three specimens were tested in the salt bridge apparatus for a duration

of 500 hours and no signs of corrosion was noticed. The corrosion resistance

of the duplex coating under service conditions as in an IC engine was studied

by visual examination after the endurance test. No signs of corrosion were

seen on the coated surfaces.

5.7 CONCLUSION

The mullite coated cast aluminum withstood a corrosive

atmosphere for more than 48 hours when tested in a salt spray chamber.

Similarly, the coating withstood the corrosive atmosphere for 500 hours,

when tested in a slat bridge apparatus due to the good corrosion resistance of

the ceramic material. Ceramics have generally a good corrosion and oxidation

resistance property and mullite is one such material. The endurance test on

road conducted on the coated engine revealed no signs of corrosion on visual

examination.