CORROCTION of metals

ABSTRACT

In order to appreciate losses accruable to engineering structures from the interaction with various environments, the report was written to give the effects of aerial ammonia on metal corrosion.

This test was conducted using nine different samples under normal atmospheric conditions at three different ammonia concentrations (1.5moldm­­­-3, 1.0moldm­­­-3, and 0.5moldm­­­-3) on three types of metal (aluminum, copper and iron).

INTRODUCTION

Most metals used by man are subjected to attack by the environment they encounter.  The attack which progresses in many cases slowly is as a result of interaction between the metallic materials and their environments. This interaction is of vital importance in the performance of materials of construction.

A common interaction herein discussed is the corrosion of metallic materials.

Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical and/or electrochemical reaction with their environment. In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfates. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal, and results in a distinctive orange colouration. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term "degradation" is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

Corrosion is defined as the destruction or deterioration of material because of reaction with its environment. Some insist there the definition should be restricted to metals but often the corrosion engineers must consider both metals and non-metals. Ceramics, rubber and other non-metallic materials, deterioration of paints and rubber by sunlight or chemicals, fluxing or lining of steel making furnace and attack of solid metal by another molten metal (Liquid metal Corrosion) are all considered to be corrosion. Corrosion can be fast or slow.   Corrosion of metal can be considered as extractive metallurgy. Rusting is a term reserved for steel and iron corrosion, although many other metals form their oxides when corrosion occurs. Practically all environments are corrosive to some degree. Air, moisture, fresh distilled, salt  and mine waters, rural or urban and industrial atmosphere, steam and other gases such as NH3, Cl2, H2S, SO2, fuel gases, inorganic acids, organic acids, solvents, petroleum oils cause corrosion environment. Inorganic materials are more corrosive than the organics. High temperature and pressure involve more severe corrosion conditions. Anodizing of aluminum is another beneficial corrosion process used to obtain better and uniform appearance in addition to a protective corrosion product on the surface.

EXPERIMENTAL PROCEDURE

1.      Standard pieces(iron, copper and aluminuim) of size 6cm by 3cm is  cut into 3 pices each, makeing a total of 9 samples.

2.      The specimens are grinded and polished to create a clean surface and prevent any form debris from interfering with the experiment.

3.      After grinding a hole is drilled into each sample to allow the sample to be suspended, to prevent the sample from resting on container which would interfere with the results.

4.      A hot condition/situation is perfectly avoided

5.      And the samples were partially immersed in the reagent solution.

6.      The samples were observed at first to recognized spots where corrosion is rapid and vigorous.

7.      After the recognitions, other samples were left for 48 hours.

8.      The masses of the metals were reweighted and recorded.

9.      The samples were then observed for the last time after 72 hours of immersion and the liquid solution was pour off into a watching-glass.

10. The liquid color was observed

11. The metal was then rinsed with water and corroded areas were inspected.

DATA AND RESULTS

1.5moldm­_­­^-3

	Initial weight(W0)	Weight after 48 hours(W2)	Weight after 72 hours (W3)
Aluminuim	4.105	4.105	4.099
Copper	18.816	16.792	14.758
Iron	12.610	11.603	11.591

1.0moldm­_­­^-3

	Initial weight(W0)	Weight after 48 hours	Weight after 72 hours
Aluminuim	3.150	3.133	3.121
Copper	13.856	13.829	13.812
Iron	11.052	11.050	11.040

0.5moldm­_­­^-3

	Initial weight(W0)	Weight after 48 hours	Weight after 72 hours
Aluminuim	4.570	4.560	4.545
Copper	14.033	13.987	13.920
Iron	11.072	11.024	11.015

DISCUSSION

Corrosion of copper, Aluminum, and steel in the form of small plate has been studied in different medium by weight loss method. Three different concentrations ( 0.1M, 0.2M, 0.05M) of NH₃  were used for different timings.

From the study the following conclusions have been arrived.

i)                   The rate of corrosion increases with increasing the concentration of the acid.

ii)                 The rate of corrosion increases with increase in time.

Corrosion rate on iron steel is the highest, followed by copper and finally by aluminum.

QUESTIONS AND ANSWERS

Why is it that iron hand-rails on the stairs rarely rust, while the under-supports of bridges do?

This question can answered by considering the kind of treatment to the iron has been subjected. As in the case of hand-rails, the iron used has been chemically treated with chromium which helps in prevent the iron from having direct contact with the oxides. However for the under-support in the bridges, the iron is used as composite, in which case oxides and other corrosion factors are made in contact with iron hence rust occurs.

Why is it that metal statues seem to shed a blue-green stain on their stone bases? What metal is used for such statues?

These blue-green strains are cause by the action of corrosion and the dissolution of the metal itself when subjected to acidic solution, water quality ( acidic water) and improper electrical grounding hasten corrosion.

Metal that exhibit this statues are copper, brass etc.

What is the white bloom that appears on the aluminum and on lead when exposed to the weather?

In actual reality, aluminum and lead doesn’t rust but when exposed to weathering the two corrode by a process called oxidation. During which a white bloom powders are formed ton their surface. The powders are the oxides of aluminum and lead respectively. The oxides appeared as coating to harden and protect the layer from corrosion.

Where does rusting first set in on corrugated galvanized iron or steel sheet, used for roofing? Why do you think it occurs there?

Corrosion first set in at the surface. This is because the metal sheet surface is exposed to the atmosphere and when it rains, the surface because moist since rain water could contain some element of acids and oxides like sulphate, nitrate and chlorides.

Methods of preventing rusting in iron or steel:

Galvanizing

COR-TEN or weathering

Alloying

Proper pre-designing

Bluing

Regular maintenance

FOZZ

Powder coating

Organic coating and etc.

 The most effective one has always been galvanizing with considerable regular maintenance.

CONCLUSION

With this result it is very clear that environment affect material a lot, as a Material Engineer corrosion is one of the area to monitor very well, the kind of environment to use a material and the kind of prevention to use in that kind of environment. Since corrosion cannot be 100% prevented but it can only be minimize.

A common way to minimized corrosion as reported herein is galvanizing anchored with regular maintenance.