Report on Structural Corrosion

Corrosion Report

Introduction

This report will explore the infrastructure corrosion contained on the campus of Strathclyde University in the city centre of Glasgow. Being one of the biggest university campuses in Scotland, it was not hard to find infrastructure corrosion. However, there are many different types of infrastructure corrosion that a building can become a victim of. Some are formed by chemical processes; others can be formed by weathering or biological growth in cracks and crevices on the walls of buildings.

Throughout this report we will look into these different types of corrosion that can be found on the Strathclyde campus and discuss the formation and impact of different examples selected from the campus.

Rottenrow Gardens

Please refer to Photo 1 in the appendix. This is an example of the weathering process – freeze thaw action corroding the side of a brick wall. In this picture it shows a heavily corroded top of the wall. Towards the bottom the corrosion doesn’t seem to be as bad. This is because the top of the wall is more subject to contact with rain and also provides a flat surface on which the water can gather, by ‘pooling’. When the rain pools at the top of the wall it is absorbed into very small cracks in the brickwork, especially where the cement meets the brick. For freeze thaw action to occur rain water must collect in these cracks. If the water is cold enough the water will freeze, causing it to expand which puts pressure on the cracks. This repetitively – occurring sequence will, over extended periods of time, increase the size of the cracks which in turn makes it more vulnerable to freeze – thaw action. This in turn causes the major cracking and deformation shown in Photo 1.

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One method to prevent freeze thaw action is foam. This is a substance that can be sprayed on the exterior of buildings. It prevents freeze thaw action by filling up the cracks in the brickwork. This prevents water entering, which in turn will stop freeze thaw action occurring. This method also insulated the interior of the building, as less heat transfer to the exterior occurs.

Due to the large number of voids created by freeze – thaw action, and due to the moisture, that can gather here, there is a natural build-up of biological vegetation. This growth of vegetation can also result in damage to infrastructure.

Another example of biological corrosion can be found on the St. Paul’s Building.

St. Paul’s Building (1)

Please refer to Photo 2 in the appendix. Biological corrosion is a very common form of corrosion found on campus. It can include vegetation such as moss and plants which grow in cracks in brickwork. Moisture can get into these cracks which contributes to the growth of vegetation. While this can be an eyesore, due to discolouration, it can also cause serious damage to infrastructure. Structural damage is accelerated by the roots of the plants growing deep into cracks along the brickwork. This widens the cracks causing the brickwork to be further exposed to weathering processes. Due to the deepening of cracks it can also make the structure weaker. It can be seen on Photo 2 that the cracks have made the brickwork so weak that it has become brittle and some parts of it have fallen off. Trying to defend a structure from biological corrosion can be difficult because regularly cleaned buildings tend to have more biological growth. This is because the layer of dirt can defend against biological growth. For example, many buildings from the industrial revolution are darkened by soot. This layer can be removed by sand blasting. However, this leaves the building exposed to living organisms and vegetation which can live in the cracks and crevasses. The cleaning of the building can have an unintended ironic effect as over time the building may appear to look green and slimy. Another form of corrosion that can be seen in Photo 2 is efflorescence. This corrosion is frequent on the St Paul’s building, as can be seen in Photo 3.

St. Paul’s Building (2)

Please refer to Photo 3 in the appendix. Efflorescence is a chemical process that leaves brickwork looking discoloured and chalk -like. It occurs when moisture invades the brickwork. The salts in the brick dissolve into the moisture. The salt is then withdrawn out of the brickwork, leaving the wall with a white stain. This is very common in Glasgow because Glasgow’s climate influences the growth of efflorescence. These factors include cold temperatures and humid air. Removing efflorescence can be complicated. It will dissolve in water easily unless it has been reacted with carbon dioxide. Before reacting with carbon dioxide pressure – washing will easily rid any concrete surface of efflorescence. After it has reacted with carbon dioxide however it becomes insoluble. To remove the reacted efflorescence the first step is to apply a mild acidic solution. An acid as simple as vinegar can be used. This may require appropriate protective clothing, depending on the size of the job. The next step is to power – wash the concrete. If the concrete is not neutralised the remnants of acid can damage surrounding flora. This acid could also further react with the concrete which can have the unintended effect of producing further efflorescence.

St. Paul’s Building (3)

Please refer to Photo 4 in the appendix. Photo 4 illustrates an example of a corroded iron pipe. Corroded iron (hydrated iron (III) oxide) has the molecular composition of 2Fe2O3.H2O. The corrosion of iron is the chemical process commonly known as ‘rusting’. This easily recognisable reddish-brown colour that iron transforms into when it is exposed to the elements is the result of three different chemical reactions.

Oxidation reactions are those in which an element loses electrons. Electrons are subatomic particles that give elements their negative charge. Therefore, losing these electrons in an oxidation reaction results in the element gaining a positive charge. The charge of a molecule is also known as its valence. As an element is oxidised its valence increases. When iron is left exposed the following oxidation reaction occurs:

Fe(s) → Fe2+ + 2e–

A reduction reaction is the opposite of an oxidation reaction. These reactions involve an element gaining electrons. The gaining of these negatively charged subatomic particles results in the element losing charge. This loss of charge can also be expressed as a decrease in valence. The reduction reaction that occurs in the formation of 2Fe2O3.H2O consists of oxygen being reduced to hydroxide ions:

O2 + 2H2O + 4e– → 4OH–

These oxidation and reduction reactions, occurring simultaneously, result in the combined chemical reaction:

2Fe + O2 + 2H2O → 2Fe2+ + 4OH–

The resultant Fe2+ and OH– ions then react together to form iron (II) hydroxide:

Fe2+ + 2OH– → Fe(OH)2

This resultant iron (II) hydroxide reacts with the water and oxygen molecules present in the surrounding air:

4Fe(OH)2 +O2 +H2O → 2Fe2O3.H2O

This final reaction produces, 2Fe2O3.H2O, hydrated iron (III) oxide. This final product is the rust that can be observed in Photo 4.

A method to prevent this rust occurring is a multi-coat paint system. The pipe has already had a layer of protective paint applied to it; however, over time this paint has worn away, most likely due to the weathering processes it has been a victim of. This method works by layering the material that is being protected in multiple layers of protective coating. At the base of the multiple coats is a layer of zinc rich epoxy primer. This allows for some conductivity, which gives cathodic protection to the pipe.

Conclusion

Finding corrosion on the University of Strathclyde campus was not hard. Many of the buildings date back many decades, so it is understandable that many of the structures have been prone to Glasgow’s harsh and constant weather. All but one of the examples used in this report were located on the St. Paul’s Building, located on the Western side of the campus. This is one of the few remaining buildings on John Street as almost all the other ones on this street have been or are in the process of being demolished. Even although the University has a permanent staff that caters to the structural upkeep of the campus, the primary way the University has combated this structural damage is demolition and rebuilding which can clearly be seen on the eastern side of the campus. The University will invest £650 million during this decade. This includes many new buildings, such as a state-of-the-art Sports Centre which has only recently opened, costing £31 million. Virtually no structural corrosion can be found on the eastern side of the campus. This refurbishment shows how dedicated our University is to the education of the next generation.

The following pages are references and appendices:

• https://chem.libretexts.org/Textbook_Maps/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Oxidation-Reduction_Reactions
• https://buildingsciencelabs.com/wp-content/uploads/2017/10/Freeze-Thaw-Homeowners-Guide.pdf
• https://www.concreteconstruction.net/products/decorative-concrete-surfaces/what-causes-efflorescence-and-how-do-you-remove-it_o
• https://www.strath.ac.uk/whystrathclyde/campusupdate/
• https://www.sciencedirect.com/topics/chemistry/efflorescence
• http://qjegh.lyellcollection.org/content/31/4/315
• https://en.wikipedia.org/wiki/Frost_damage_(construction)
• http://tdws.nwasco.k12.or.us/staff/bfroemming/Corrosioniron.html
• CL218 – Lecture 6 Slides