Corrosion happens to be the primary cause for maintenance related issues wherever metal is used. Even protected metal infrastructure, such as pipes, can become exposed to over time and start corroding.

Two chemical processes are involved in corrosion, namely, oxidation and reduction. Oxidation takes place at the anode and involves the stripping of electrons from an atom while reduction takes place at the cathode and involves the addition of an electron to an atom. This electrochemical process in pipelines can lead to corrosion and eventual failure of the pipe.

Corrosion Rate and Corrosion Index

Sewer pipes made of concrete and metal are prone to corrosion because of the release of hydrogen sulfide gas from the wastewater. Engineers can now calculate the serviceable life of a material and decide which pipe material will be most suitable for a particular condition by calculating the rate at which a material will corrode when exposed to certain conditions.

The corrosion rate is calculated by determining the material density, loss in material weight, time period leading to the loss in weight and cross-sectional area of the corroded region. It is expressed in mils per year (mpy) or milligrams per square decimeter per day (mdd).

Galvanized pipes carrying water can become corroded if the water has an unbalanced pH. Corrosion index is a method used to determine whether the water that the pipe is transporting will cause corrosion or scale formation. The Langelier Saturation Index (LSI) is used to calculate the corrosion index. It takes into account pH, conductivity, total dissolved solids, alkalinity and hardness of the water and is calculated as the difference between the actual and theoretical pH of the water. The index value of 0 indicates balanced water; index of -5 indicates corrosive water and index of +5 indicates water that has scale-forming tendencies.

Types of Corrosion

Some of the corrosion categories based on the appearance of the corroded material are:

General Corrosion

General corrosion in which there is more or less uniform distribution of corrosion over the surface of the metal.

Localized Corrosion

Localized corrosion in which the corrosion is confined to a small area and is usually caused due to a concentrated cell created by the concentration of some components in the electrolyte. This leads to the formation of distinct anodic and cathodic regions. Examples are pitting, crevice and filiform.

Intergranular Corrosion

Intergranular corrosion is a type of corrosion which takes place at the metal boundary. A type is exfoliation corrosion that happens along the grain boundaries of the metal causing flaking and lifting. It usually happens at exposed parts of the metal that are not painted or sealed.

Galvanic Corrosion

Galvanic corrosion takes place when two dissimilar metals with a common electrolyte or two similar metals with dissimilar electrolytes have a spontaneous chemical reaction resulting in an electron transfer in a redox reaction.

Environmental Cracking

Environmental cracking is fracture caused to a metal due to the environment to which it is exposed, including temperature, chemicals and loading. Examples are stress corrosion cracking, hydrogen induced cracking and corrosion fatigue which is the failure of a metal due to corrosion and cyclic loading.

Causes of Corrosion in Plumbing

Old houses typically have corrosion loving galvanized iron (GI) pipes or copper pipes in their plumbing system. However, there are many reasons why pipes can become corroded.

Water pH Level

Low or high pH levels of the water can cause corrosion in pipes by harming its protective barrier.

Oxygen in Water

Excess oxygen in the water can also cause metal degradation by rust formation leading to deposits on pipe walls, creating blockages.

Minerals in Water

Though some added minerals are helpful in water, high mineral levels such as that of calcium can cause build up inside the pipe.

Water Temperature and Velocity

Water at high temperatures can worsen the problem of corrosion while high water pressure/ velocity and sudden direction changes such as through elbows and bends can cause erosion of pipe material due to turbulence.

Effects of Corrosion on Plumbing

Tuberculation

Tuberculation is the formation of small deposits of rust scattered on the inside surface of the pipe. As it progresses the interior surface becomes rough causing decrease in flow characteristics. The growth of tubercules is steady and they ultimately merge causing considerable reduction in inside diameter. (See "Cleaning and Repairing Tuberculation in Pipes.")

Low water pressure

Owing to build up inside the pipe, a decrease in water pressure is inevitable. This may be coupled with a reddish brown discoloration and rusty tasting water. (Read "How to Tell When a Water Line Needs Repair: Trenchless for Water Lines.")

Leaks and Cracks

Often corroded pipes crack due to the buildup of stress. This can lead to leaks in the plumbing system which may go undetected for some time because these cracks may have happened in pipe sections that are hidden in walls or floors. This can cause extensive damage to the section of the wall or floor through which it passes.

High Water Bills

Signs of change in the water pressure, color and taste can be an early indicator of a leak. A change in the expected water bill can also indicate leakage. This can be tested by turning off all water taps and checking if the water meter is still recording usage.

Lead Leaching

Lead leaching can be a serious health hazard. Mixing of copper and GI pipes accelerates corrosion leading to leaching of lead into the water. This is a common problem in old houses where parts of lead pipes were replaced with GI pipes and later with copper pipes. (Read on in "Using Trenchless Technology to Replace City Lead Pipes.")

Solving the Corrosion Problem

One of the best ways to eliminate the problem of corrosion is to replace pipes that are causing it in the first place. Cathodic protection can also be applied to pipelines to protect against corrosion. Regular maintenance and checking of pipelines using video inspection can detect if cleaning is enough or replacement is necessary.

Pipe materials such as GI, cast iron (CI), concrete etc. are prone to corrosion over time. New age pipe materials such as polyethylene (PE), high-density polyethylene (HDPE), polyvinyl chloride (PVC), corrosion resistant E-Glass and fiberglass reinforced plastic (FRP) are now available that are corrosion resistant, strong and have a lifespan varying from 50 to 70-years. (Read "The Lifespan of Steel, Clay, Plastic & Composite Pipes.")

Replacing plumbing is a costly affair but thanks to trenchless rehabilitation methods such as cured-in-place pipe (CIPP), mechanical spot repair, fold and form pipe, sliplining, thermoformed pipe and pipe bursting, that is no longer the case. Trenchless rehabilitation eliminates the need to dig and replace, instead it helps by forming a new pipe within the old pipe without removing it.

The convenience of using these methods are revolutionary because not only does it eliminate digging up entire pipe sections, it also brings down the cost considerably. This is because repairs can be carried out in a targeted manner in spots that are damaged or by repairing particular pipe sections. It is advisable to replace old pipes with corrosion resistant materials using trenchless rehabilitation methods before they become a problem, especially in very old houses which may even contain sections of lead pipes.