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AKINPELUMI K.F 100401013

To investigate the effect of pH level on corrosion rate

A

Short Laboratory Report

Presented To

The Department of Chemical Engineering

BY

AKINPELUMI, K.F. (Group B)

Mat No. 100401013

In Partial Fulfillment

Of the Requirements for the Course

Chemical Engineering Laboratory II

University of Lagos, Lagos

June 28, 2012


ABSTRACT

This experiment was carried out to investigate the effect of pH level on corrosion rate. The necessity of this experiment arises from the need to demonstrate how a potentially corrosive situation may be recognized and avoided.

Three sample metals – Copper, Aluminum and Stainless Steel; were dipped in 3 beakers containing – Saturated NaOH solution (pH 11), Distilled Water (pH 7) and Conc. H2SO4 Acid (pH 3) respectively.

The weight loss after a particular time interval was recorded and the corrosion rates of each individual metal in the different solutions were also calculated and analyzed.

The nature of the metal rods and solutions before and after the corrosion was observed and in certain solutions, vigorous reactions with heavy cloudiness were noticed.

It was concluded that the metals showed unique corrosion characteristics in the various solutions of stated pH levels; with aluminum showing the highest tendency to corrode under any particular pH level.


INTRODUCTION

The aim of this experiment is to investigate the effect of pH level on corrosion rate. The necessity of this experiment arises from the need to demonstrate how a potentially corrosive situation may be recognized and avoided.

pH, originally defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909, is a measure of the concentration of hydrogen ions. The term pH was derived from the manner in which the hydrogen ion concentration is calculated, it is the negative logarithm of the hydrogen ion (H+) concentration:

pH = -log [H+] …………………………….eqn 3.1

Where log is a base-10 logarithm. According to the Compact Oxford English Dictionary, the "p" stands for the German word for "power", potenz, so pH is an abbreviation for "power of hydrogen".

A higher pH means there are fewer free hydrogen ions, and that a change of one pH unit reflects a tenfold change in the concentrations of the hydrogen ion. For example, there are 10 times as many hydrogen ions available at a pH of 7 than at a pH of 8. The pH scale ranges from 0 to 14. A pH of 7 is considered to be neutral. Substances with pH of less than 7 are acidic and substances with pH greater than 7 are considered to be basic.

Low pH acid waters clearly accelerate corrosion by providing a plentiful supply of hydrogen ions. Although even absolutely pure water contains some free hydrogen ions, free carbon dioxide in the water can multiply the hydrogen ion concentration many times. When carbon dioxide dissolves in water, it reacts with the water to form carbonic acid, a so-called weak acid, but an effective source of acidity. Even more acidity is sometimes encountered in acid mine waters, or in those contaminated with industrial wastes.

Both acids and alkali's have the capability of being corrosive, although one would have a pH range of 0 - (acid), while the other would range in the area of 14 (alkali). Sodium hydroxide, a very strong and corrosive alkali would have the same damaging effect on human tissue as sulfuric acid. If a 25% concentration of sulfuric acid and phosphoric acid were measured for pH, both would range in the area of 0. However, if sulfuric acid were allowed to contact human tissue, severe burns would result, while the average person would not detect even a burning sensation from contact with the phosphoric acid. Why? The answer lies in the corrosive nature of some acids over others.

http://www.corrosion-doctors.org/Principles/Theory.htm

http://www.corrosion-doctors.org/Principles/Theory.htm

http://www.corrosion-doctors.org/Water-Glossary/Glossary.htm

http://www.corrosion-doctors.org/Hydrogen/Hydrogen.htm

http://www.corrosion-doctors.org/Hydrogen/Hydrogen.htm


Through experimentation and testing over the years by chemists, certain characteristics have been observed in the reactions of acids and alkalis, and were assigned classifications accordingly. One of the classifications is corrosiveness. Therefore to classify a product or compound as being corrosive means that it would have the potential to eat away at something, in some cases very rapidly, and it would have the capability of being harmful to objects such as metals, structural components of an aircraft and human tissue.

Corrosion is the gradual destruction of material, usually metals, by chemical reaction with its environment. 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.

http://www.lenntech.com/Periodic-chart-elements/Fe-en.htm


In this experiment, the metals provided were cut into small sizeable shapes and the solutions carefully prepared. The NaOH pellets provided were dissolved in 50 ml of distilled water, and more NaOH pellets were added till the solution could no longer dissolve the pellets and became saturated. It must be stressed here that so many pellets had to be added because NaOH is very soluble in water as a result of the hydrogen bonds formed.

Also, equal amounts of Distilled water and conc. H2SO4 Acid were shared into the remaining beakers.

Careful observations were made as to which solution turned cloudy and what the nature of the metals looked like during and after the corrosion process. The weight loss of the metals was calculated by subtracting the initial weight before the corrosion process from the final weight after washing and drying of the metal.

The corrosion rate for each of the metals in the various solutions of stated pH levels was calculated and necessary deductions were made.


RESULTS

Beaker A contained Saturated NaOH solution (pH 11)

Beaker B contained Distilled Water (pH 7)

Beaker C contained conc. H2SO4 (pH 3)

The following tables show the weight loss of the various metal samples – Copper, Aluminum and Stainless steel in the 3 beakers – A, B and C.

Table 4.1

Beaker A - Saturated NaOH solution (pH 11)

METAL SAMPLE

INITIAL MASS(g)

FINAL MASS(g)

WEIGHT LOSS(g)

TIME TAKEN(s)

Copper 12.83 12.82 0.01 1800

Aluminum 7.22 7.10 0.12 1800

Stainless Steel 10.30 10.29 0.01 1800

Table 4.2

Beaker B – Distilled Water (pH 7)

METAL SAMPLE

INITIAL MASS(g)

FINAL MASS(g)

WEIGHT LOSS(g)

TIME TAKEN(s)

Copper 11.43 11.43 0.00 3600

Aluminum 15.11 15.02 0.09 3600

Stainless Steel 9.67 9.66 0.01 3600

Table 4.3

Beaker C – Conc. H2SO4 Acid (pH 3)

METAL SAMPLE

INITIAL MASS(g)

FINAL MASS(g)

WEIGHT LOSS(g)

TIME TAKEN(s)

Copper 10.72 10.71 0.01 1800

Aluminum 7.60 7.59 0.01 1800

Stainless Steel 15.11 15.10 0.01 1800


Corrosion rate = Weight losstime taken

For Copper in Beaker A;

Corrosion rate = 0.011800

= 5.556 * 10-6g/s

For Aluminum in Beaker A;


= 6.667 * 10-5g/s = 66.667 * 10-6g/s

For Stainless Steel in Beaker A;


= 5.556 * 10-6g/s

For Copper in Beaker B;


= 0.00g/s

For Aluminum in Beaker B;


= 2.500 * 10-5g/s = 25.000 * 10-6g/s

For Stainless Steel in Beaker B;


= 2.778 * 10-6g/s

For Copper in Beaker C;


= 5.556 * 10-6g/s

For Aluminum in Beaker C;


= 5.556 * 10-6g/s


For Stainless Steel in Beaker C;


= 5.556 * 10-6g/s

Table 4.4

METAL SAMPLE WEIGHT LOSS(g)

TIME TAKEN(s)

CORROSION RATE(g/s)

Beaker A - Saturated NaOH solution (pH 11)

Copper 0.01 1800 5.556*10-6

Aluminum 0.12 1800 66.667*10-6

Stainless Steel 0.01 1800 5.556*10-6

Beaker B – Distilled Water (pH 7)

Copper 0.00 3600 0.00

Aluminum 0.09 3600 25.000*10-6

Stainless Steel 0.01 3600 2.778*10-6

Beaker C – Conc. H2SO4 acid (pH 3)

Copper 0.01 1800 5.556*10-6

Aluminum 0.01 1800 5.556*10-6

Stainless Steel 0.01 1800 5.556*10-6


DISCUSSION

From the results I obtained from this experiment, I observed that the various metals – Copper, Aluminum and Stainless Steel, displayed unique corrosion characteristics in the various pH solutions – saturated NaOH solution (pH 11), Distilled Water (pH 7) and conc. H2SO4 acid (pH3).

From the tables, I observed that Copper had the same weight loss of 0.01g after 30 minutes (1800s) in both saturated NaOH solution of pH 11 and conc. H2SO4 acid of pH 3; thereby having

the same corrosion rate of 5.556*10-6g/s in both solutions. However when placed in the distilled water for 1 hour (3600s), there was no weight loss, indicating that corrosion did not take place.

I also observed that the Copper metal that had been placed in the distilled water solution remained very much the same in terms of color and surface roughness after the 1 hour (3600s) had elapsed. Also, there was no visible reaction or effervescence and the solution remained clear throughout the 1 hour that the copper atom was in the distilled water. However, in both the saturated NaOH solution and conc. H2SO4 acid, a little cloudiness of the solutions was observed.

From the tables, I observed that aluminum had a very high weight loss of 0.12g in saturated NaOH solution of Ph 11 after 30 minutes (1800s), and also very high weight loss of 0.09g in distilled water of ph 7 after 1 hour (3600s), and a weight loss of 0.01g in conc. H2SO4 acid after 30 minutes had elapsed.

I also observed a very visible and vigorous reaction in the aluminum metals that were placed in both the saturated NaOH solution and distilled water and the final solutions became very cloudy showing evidence of corrosion.

After the metals had been dissolved and dried, there was a visible shrinking of the metal and it appeared to have been charred, though this could have been as a result of some impurities in the metal.


The aluminum metal in the acidic solution did not show too much visible reaction though there was a little cloudiness.

From the tables, I observed that stainless steel had the same weight loss of 0.01g in both saturated NaOH solution of pH 11 and conc. H2SO4 of pH 3 after 30 minutes and 0.01g in distilled water of pH 7 after 1 hour. This shows that irrespective of the pH level, stainless steel had a relatively constant corrosion rate.

Also I observed that in all of the 3 solutions, there was a little cloudiness and the stainless steel metal showed a little shrinking.

The data obtained can therefore be summarized in the table below;

Table 5.1

METAL SAMPLE

CORROSION RATE(g/s)

Saturated NaOH solution(pH 11)

Distilled Water(pH 7)

Conc. H2SO4 Acid(pH 3)

Copper 5.556*10-6 0.00 5.556*10-6

Aluminum 66.667*10-6 25.00*10-6 5.556*10-6

Stainless Steel 5.556*10-6 2.778*10-6 5.556*10-6


CONCLUSION

The corrosion rate of Copper metal in a high pH level solution of 11 is the same as in a

low pH level solution of 3 and has a value of 5.556*10-6 g/s.

Also, in a completely neutral solution of pH 7, no corrosion occurs in the copper metal after 1 hour (3600s).

The corrosion rate of Aluminum is found to vary directly with the pH level.

As the pH level increases from 3 to 11, the corrosion rate of Aluminum is found to show a relatively proportional increase.

Therefore, it can be implied that corrosion is more visible in aluminum metal when it is dipped in solutions of higher pH values.

The corrosion rate of Stainless Steel in a high pH level of 11 is the same at a low pH level

of 3 and has a value of 5.556*10-6 g/s.

Also, in a completely neutral pH of 7, minimal corrosion occurred in the Stainless Steel

after 1 hour (3600s); with a corrosion rate of 2.778*10-6 g/s.

Also, at a low pH level of 3, all the 3 metals – Copper, Aluminum and Stainless Steel have

the same corrosion rate; a value of 5.556*10-6g/s.

At a high pH level of 11, Aluminum had the highest corrosion rate of 66.667*10-6g/s

while copper and Stainless Steel had the same corrosion rate of 5.556*10-6g/s.

The various metals – Copper, Aluminum and Stainless Steel, displayed unique corrosion characteristics in the various pH solutions – saturated NaOH solution (pH 11), Distilled Water (pH 7) and conc. H2SO4 acid (pH3)


From all the sample metals tested, Aluminum showed the highest tendency to corrode under any particular pH level condition.

RECOMMENDATIONS

I would suggest that a more efficient way of immersing or dipping the metals into the respective solutions is adopted. The use of strings is strongly recommended as this would expose a larger surface area of the metal to the corrosive activity of the solution.

I suggest that the aluminum metals be thoroughly scratched after the experiment before they are being used for any subsequent experiments because the aluminum metals charred and had a coating of black soot around them.


APPENDIX

To further corroborate my conclusion that the corrosion of the aluminum metal increased with increasing pH level, the conc. H2SO4 acid used was diluted with water to increase the pH level, and there was a much more noticed vigorous reaction, with the solution becoming very cloudy; implying that corrosion was much more pronounced at that higher pH level of the diluted acid.

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