Corrosion. We've all heard of it, seen pictures of the damage it can do, and some of us may have even experienced it in our homes, schools, or places of work. But do we really know how it works, and the extent of the damage it can do? Read on and find out!



Corrosion

Corrosion is a natural redox process that oxidizes metals to their oxides and sulfides. In chemical terms, corrosion is the reverse of isolating a metal from its oxide or sulfide ore; in electrochemical terms, the process shares many similarities with voltaic cells. Damage from corrosion to cars, ship, buildings, and bridges runs into tens of billions of dollars anually, so it is a major problem in much of the world.

The most common and economically destructive form of corrosion is the rusting of Iron. Rust is not a direct product of the reaction between iron and oxygen but arises though a complex electrochemical process.

Facts of Iron corrosion:

1) Iron does not rust in dry air: moisture must be present.
2) Iron does not rust in air-free water: oxygen must be present.
3) The loss of Iron and the depositing of rust often occur at different places on the same object.
4) Iron rusts more quickly at low pH (high [H+]).
5) Iron rusts more quickly in contact with ionic solutions.
6) Iron rusts more quickly in contact with a less active metal (such as Cu) and more slowly in contact with a more active metal (such as Zn).

external image rusting.gif
Chemistry Involved in Corrosion.


(Fact 1). These sites are called anodic regions because the following half reaction occurs there:
....................................Fe(s)----> Fe2+(aq)+ 2 e- [anodic region: oxidation]
When the iron atoms lose electrons, damage to the object had been done, and a pit forms where the iron is lost.
The freed electrons move through the external circuit (the piece of iron itself) until they reach a region of relatevilely high )2 concentration (fact 2), near the surface of of a surrounding water droplet, for instance. At this cathodic region, the electrons released fromm the iron atoms reduce )2 molecules:
................................... O2(g) + 4H+(aq) + 4e------> 2H2O(I) [cathodic region, reduction]

In many ways, the components of the corrosion process resemble those of a voltaic cell:
........* Anodic and cathodic regions are separated in space.
........* The regions are connected via an external circuit though which the electrons travel.
........* In the anodic region, iron behaves like an active electrode, whereas in the cathodic region, it is anactive.
........* The moisture surrounding the pit functions somewhat like a salt bridge, a means for ions to ferry back and forth and keep the solution neutral.




Effects of Corrosion on Drinking Water

The corrosion of metal pipes responsible for the transport of our drinking water can become a hassle quickly if it is not kept in check. Often, pipe corrosion leads to tuberculation (mounds of rust built up inside a pipe). The inner corrosion/rust of a water pipe can break free and enter the distribution system for drinking water. In addition, if the plumbing fixtures in the house contain lead or copper, a more common occurrence in houses built before 1981, drinking water can become contaminated with corroded metals as well. Corrosion in drinking water causes various problems, including lesser aesthetic quality of water, stained laundry/plumbing fixtures, and hazardous short and long term health effects.
corrosion-interior-pipes.jpg
Original picture retrieved from http://www.thewatertreatments.com/corrosion/effects-corrosion-scaling

Effects of corrosion on drinking water are as follows…
Appearance of Drinking Water:
The elevated levels of different metals such as lead, copper, chromium, and zinc can leave corrosive drinking water with a bitter taste, lead to red water (reddish tint due to rust), stain laundry and leave greenish-blue stains on drains and plumbing fixtures.
Health Hazards from Corrosive Water:
The presence of these toxic metals is also a cause for concern because of the related health risks that arise from drinking corrosive water. Severe effects from drinking corrosive waters include: cancer, organ damage and joint disease, damage of the kidneys, circulatory system and nervous system, and lead poisoning. In some cases, it can cause autoimmunity, where the immune system attacks its own cells. Youth are especially susceptible, more prone to toxic effects and more sensitive. Due to these dangers, the Federal-Provincial-Territorial Committee on Drinking Water(CDW) enforces guidelines for Canadian drinking water quality, in an effort to avoid these detrimental effects. More on the CDW guidelines in the link below…
……….>http://www.hc-sc.gc.ca/ewh-semt/water-eau/drink-potab/guide/index-eng.php

Types of corrosion:
There are many types of corosion that may affect important pipes. All of them are very destructive, and should be prevented if possible.
Follow this link to learn more!
...........> http://www.corrview.com/corr_06.htm

Common methods of corrosion prevention and its impacts on human and environmental health.

1) Painting - Corrosion can be prevented by applying a layer of paint over the surface. The layer of paint applied acts as an insulator and as a barrier between the underlying metal and oxygen. Therefore oxygen cannot come in contact eith the metal and the reaction would not occur. So the metal would not corrode. The paint may wear away with time but a new layer can always be applied. This method is usually used by automobile industry to protect cars from corrosion. Even though painting the metalic substance is a great method of corrosion prevention, it also has negative impacts. When paints dry up, they release VOC (Volatile Organic Compounds). They can have harmful effects on humans and environment. The fumes of VOC are very toxic and can cause problems when inhaled and may play a role in pollution.

2) Galvanizing - It is a process of coating a material over the substance that needs to be prevented from corrosion. Galvanizing is performed by using a chemical process to form a chemical bond between the actual material and the other substance that will act as cover. Galvanizing is more effective than painting the surfaces as it lasts longer and doesn't have any harmful impacts on the environment and human beings.

3) Cleaning - Corrosion can also be prevented by keeping the surface clean and thus preenting it from chemicals. It is not a very effective way of prevention but is accepted in some cases.
4) Cathodic Protection - This method of corrosion prevention works only with iron. We all know that oxidation is the loss of electrons and occurs at anode of a cell. Oxidation occurs when a substance loose electrons. Iron acts as an anode and therefore loses electrons. Iron can be prevented from corrosion if we can act iron to behave as a cathode. We can use a DC generator and connect its negative terminal to iron. Therefore electrons will flow into iron and it is forced to behave as a cathode. This will prevent corrosion.

Problems Within its Structure
What is corrosion caused or accelerated by?
  • damp/high relative humidity
  • pollution (external sources - sulphur dioxide produced by car exhaust, fires, smoking; internal sources - organic acids given off by wood and wood-based products, fresh paints and lacquers)
  • many general household cleaning products contain corrosion-inducing chemicals. Aerosols can leave tiny droplets which are invisible to the naked eye, but, given time, may damage a coating or the metal itself
  • display or packing materials that give off sulphur or acidic vapours (e.g. wool or felt linings, some plastics, such as PVC or rubber, newspaper and some adhesives)
  • contact with acidic parts, such as leather components (in combination with high relative humidity)
  • food residues (e.g. acidic residues from vinegar or fruit juices; salt left in salt cellars) residues from metal cleaning products (e.g. proprietary metal polishes that contain ammoniates)
  • dirt and dust accumulation (which can attract moisture and hold it on the surface)
  • galvanic corrosion - where two metals are in contact or are alloyed, one may corrode faster than the other
  • coatings that do not completely cover a surface - the uncoated areas will corrode faster than if no coating had been applied
So, with that in mind what is being effected within the structure?
As said above, Corrosion is the deterioration a material undergoes as a result of its interaction with its surroundings. All metals will have different physical, chemical, and mechanical properties, but all of them can corrode to some extent, and in different ways. Therefore, Corrosion processes influence the chemical properties of a metal, generate changes in its physical properties and its mechanical behavior. This is why the effects of corrosion are manifested in a variety of forms.

Safety Issues
Corrosion control and public safety go hand-in-hand. Corrosion is one of the leading causes of infrastructure related accidents in the world. When a pipeline or structure fails, the results can be devastating in terms of loss of life, property damage, and contamination of the environment. To prevent such disasters, private firms and governments have been using corrosion control methods as one of their strongest weapons in the battle against corrosion failures.

See this link http://www.washingtonpost.com/wp-dyn/content/article/2010/11/02/AR2010110207708.html?wprss=rss_nation&sid=ST2010110303032

After reading this, we hope you've learned the risks of corrosion on health and the public in general.

Sources: