Corrosion and Mitigating it with CPVC

Corrosion in itself is a dangerously expensive industry. The annual global cost of corrosion is $2.5 trillion. $23 billion is spent a year on preventing corrosion in the world’s structures, industries and products.

Not all corrosion is due to harsh chemicals; for example, external cladding does not corrode from exposure to chemicals; a metal structure built in a coastal town is naturally susceptible to the salt in the air.

But not all materials suitable for cladding, fencing and construction are as susceptible to corrosion. TempRite^® Engineered Materials have engineered CPVC to resist most types of corrosion that happen in the atmosphere.

Uniform Corrosion - i.e. rusting. The gradual receding of the entire surface area of a metal through exposure to natural materials in air, soil and water.

Crevice and Pitting Corrosion - This happens in crevices where there is a barrier, such as two metal parts fastened together. Oxygen is prevented from entering gaps and pairing with metal ions to form a natural oxide layer of protection. Without this molecular pairing, acid is formed, with nothing to stop it.

Erosion Corrosion - Erosion corrosion happens when turbulent water puts too much stress on the surface, knocking away the passivated surface layer.

Stress Corrosion Cracking - Stress Corrosion Cracking is frequently at fault for collapsing buildings or bridges and metal pipe failure. Strain on the structure exposes underlying metal to degradation, causing cracks to deepen in very localised areas.

Stress Corrosion Cracking is the only form of corrosion to which CPVC is vulnerable, and that only occurs in the presence of extremely corrosive chemicals and high levels of strain.

Corrosion Causes and Prevention

In order for corrosion to take place, a material must come into contact with specific substances or environments. Unfortunately for engineers, many of these are part of the natural world:

• Oxygen combined with  water
• Pollutants
• O3, or ozone
• Salt air
• Bacteria

CPVC is inert to all of the environmental factors that can corrode metals. For example, metal fire sprinklers which contain stagnant water can accommodate bacterial build-up. Bacteria excretes acid which can cause pitting corrosion beneath bacterial colonies. CPVC is highly resistant to bacterial colonisation, and also highly resistant to the acid excreted by bacteria, should any colonies manage to form.

There are various types of protection employed by manufacturers to prevent the effects of corrosion.

• Coatings
• Humidity control
• Corrosion inhibitors
• Cathodic protection

These can be expensive and difficult to maintain, and faults in the protection can leave the metal vulnerable. CPVC’s inherent corrosion resistance makes this trouble and expense unnecessary.

Why Does Corrosion Happen?

Atoms are at their most stable when their orbitals are fully occupied with electrons.

Metals have an outer orbital which is only about half full. They therefore crystallize and pool their spare electrons. This electron pooling is what gives metal malleability and ductility, allowing manufacturers to flatten metal into sheets and draw it into wires. It is also what makes metals conductive.

However, this leaves them open to attack. Oxygen is everywhere in the atmosphere, and prefers not to share electrons, it prefers to take. Oxygen targets metals for its pool of electrons and claims them, oxidizing the metal and forming corrosion. This is why iron perpetually corrodes away over time; however, for other materials, there is one line of defense.

Passivation

Metal oxide forms stable crystals on the outer surface of most metals. As long as the surface is not disturbed, it will serve as a protective barrier from any continued corrosion. Iron is unable to form a passive surface, and so it rusts. Iron can be made to passivate by forming alloys; for example, chromium is added to stainless steel so that the chromium will form the passive layer, protecting the iron.

An Easier Way

Beyond choosing metal for its obvious mechanical properties, there must be a smarter choice when it comes to specifying materials for construction or manufacturing. Is plastic the alternative?

PPR and Polyethylene are very vulnerable to corrosion by oxygen or strong acids. They employ antioxidants to counteract this, however they are purely sacrificial and eventually deplete. PVDF is expensive and pH limited; even mild caustic applications can cause corrosion.

Thanks to CPVC, accepting the expensive risk of corrosion can be avoided.

Engineering CPVC for Corrosion Resistance

In the chlorination process that makes CPVC resins, UV and high temperatures are employed to activate chlorine and attach it to the molecular backbone, forming a protective layer of atoms which shields the polymer from attack by corrosive chemicals.

 

CPVC’s natural resistance to corrosion is obvious from the very method used to manufacture the resin; if high levels of chlorine and ultraviolet light degraded CPVC, we simply wouldn't be able to make it.

This robust resin is naturally equipped to resist corrosion from some of the harshest acids, caustics and oxidizers, including:

• Sulfuric acid
• Nitric acid
• Caustic soda/potash
• Hydrogen peroxide
• Industrial bleach
• Chlorinated water

  Sulfuric Acid Resistance