Lesson 13:
Corrosion

 

In this lesson we will answer the following questions:

• What is corrosion?
• What are some factors affecting corrosion?
• How do you prevent corrosion from occurring?
  

Lecture

Introduction

From general experience of life, we probably have some idea about what corrosion is and have experienced the higher levels of corrosion that occur in the presence of moisture and the hostile gas species, such as chlorine, ammonia, sulphur dioxide, hydrogen sulphide and oxides of nitrogen, that are often present in the atmosphere. Whether our experience is of tarnished silver or rotting exhaust pipes, what we have seen is corrosion, defined as "the destructive attack of a metal caused by either a chemical or an electrochemical reaction with the various elements in the environment".

The phenomenon of corrosion involves reactions which lead to the creation of ionic species, by either loss or gain of electrons. Take the case of the rusting of iron, where metallic iron is converted into various oxides or hydroxides when exposed to moist air. The equations for this reaction are:

2Fe_(solid) + 2H₂O_(liquid) + O_{2 (gas)} → 2Fe²⁺ + 4OH^- → Fe(OH)_{2 (solid)}

 

2Fe(OH)_{2 (solid)} + H₂O _(liquid) + ½O_{2 (gas)} → 2Fe(OH)_{3 (solid)}

 

Because the iron loses electrons, chemists refer to this as an oxidation process. In this case, the resulting product is a mix of hydroxides; where copper reacts with acid residues creating chlorides and sulphides, this would also be referred to as an oxidation process.

 

 

Factors Affecting Corrosion

Corrosion of bare conductors will happen at a rate that varies substantially, depending on the conditions. The table below indicates some of the factors that affect such corrosion. Note that this is for unprotected materials, and that the addition of an effective surface coating can affect the outcome for the better.

Factors That Affect Corrosion
Conductors	Nature of the material or alloy Surface condition/roughness Conductor configuration Conductor-conductor spacing
Substrate	Composition Moisture absorptivity Structure Nature of any reinforcement
Environment	Temperature Humidity Corrosive elements

 

Another factor that affects the rate of progress of corrosion is the nature of the "corrosion product". If the material produced by corrosion is insoluble and forms an impervious and tenacious layer, the corrosion reactions becomes self-limiting, as the corrosive medium can no longer diffuse through the corrosion product. A useful example of this is the oxidation of aluminum, which forms a thin protective layer of aluminum oxide, this is a good example of self-passivation.

If, on the other hand, the corrosion product is soluble or porous, corrosion will continue until the material is depleted, and no further reaction can occur. This is seen with the rusting of iron, where the oxide/hydroxide "rust" has a different crystal structure from the iron, and creates only a porous, poorly adherent layer which does not protect against continued attack.

 

 

Types of Corrosion

Below is a list of the various types of corrosion.

Uniform corrosion The reation starts at the surface and proceeds uniformly.	Localized corrosion (pitting corrosion) The basis metal is eaten away and perforated in places in the manner of holes, the rest of the surface being affected only slightly or not at all.	Wide Pitting corrosion The corrosion causes localized scarring.
Intergranular corrosion Imperceptible or barely perceptible from outside, since the corrosion proceeds at the grain boundaries.	Transgranular corrosion The grain boundary material is retained, since the corrosion proceeds preferentially within the grain.	Galvanic corrosion Increased corrosion in crevices or cracks or at contact surfaces between two metal articles.
Selective corrosion Corrosive attack on structural constituents	Exfoliation corrosion Occurs in deformed articles. Corrosion follows "fiber orientation"	Interfacial corrosion Frequently observed at water-air interfaces

 

 

Corrosion Causes and Treatment

Corrosion of distribution lins, home plumbing and fixtures has been estimated to cost the public hundreds of millions of dollars per year. Lead and cadmium, both toxic metals, occur in tap water almost solely due to corrosion. Three other metals, usually found in high concentrations due to corrosion of piping systems are copper, iron, and zinc. Copper causes blue staining and imparts a metallic bitter taste. Iron corrosion causes reddish or brown water and also imparts a metallic taste. Zinc corrosion does not usually discolor the water, but can also cause a metallic taste. All waters are corrosive to some degree, and a water's corrosive character depends on its physical and chemical constituents. The type of material the water comes in contact with also affects the "corrosivity". For instance, water that may corrode iron pipe may not be as corrosive to copper pipe.

In our area, the most common causes of corrosion of plumbing systems on municipally treated water is from "electrolysis" or electrically induced corrosion. This is often due to improper grounding of electrical systems to water pipes, or by the creation of "galvanic corrosion cells" in home systems due to the dual use of iron and copper piping in the same system. This "electrically-induced" corrosion is aggravated by waters high in total dissolved solids, making the water more conductive.

On private water systems, the most common cause of corrosion is from low pH (less than 7.0) waters. Often these waters are of high quality and are low in buffering calcium minerals, but are high in dissolved carbon-dioxide gas, which can cause the low pH or acidity. Treatment is accomplished by neutralizing the water with the use of an automatic neutralizer. These water filter tanks are filled with a blend of calcium and magnesium carbonates made from naturally occurring minerals, which dissolve into the water, making it less corrosive. Other methods commonly used are pH adjustment by injecting soda ash or a sodium hydroxide solution into the water upstream of a holding or retention tank.

 

 

Review

Corrosion is the destructive attack of a metal caused by either a chemical or an electrochemical reaction with the various elements in the environment. There are various types of corrosion that affect different elements. Lead and cadmium, both toxic metals, occur in tap water almost solely due to corrosion. Three other metals, usually found in high concentrations due to corrosion of piping systems are copper, iron, and zinc. Corrosion can often be treated by raising the pH of the water or by adding phosphate in various forms into the water to seal off the corroding piping.

 

 

Sources

Transport Information Service. Types of Corrosion.

University of Bolten. Corrosion.

Advanced Water Systems. Water Problems - Corrosion.

 

 

Assignments

Complete the interactive exercises in Assignment 13. 

You may do the Assignment online to get credit or print it out and send it to the instructor.  It will require the Flash flayer to view, which should already be installed on your machine.

 

 

Lab

Read the Iron and Calcium Carbonate Saturation labs.

 

 

Quiz

Answer the questions in the Lesson 13 quiz .  When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.