Basics of Corrosion on Carbon Steel

Corrosion is a Chemical Reaction

The typical causes are influenced by the 'half and half-not'  Simply, stating that when there are differences there is a potential for corrosion.  In chemistry we refer to this as electro-chemical reaction but in but in the world of metals corrosion becomes more complicated but operates on the same fundamentals as electro-chemical reactions.  Here are some examples of differences that will cause basic corrosion-

Electrical differences between the two-metals.  This is sometimes referred to as the galvanic series.  Magnesium and zinc are at one end or the most negative side and graphite and platinum are at the other end or the most noble (positive) side.

If the metal is more impervious of the attaching substance that could be a chemical solution that is either acid or alkaline.  Carbon steel likes a slightly alkaline solution.  When the pH of the metal and solution are not different or are the same then there will be no corrosion.

Oxygen has another way of causing a chemical reaction.  In fact, as you may remember early in the tutorial iron or carbon steel all originate from iron ore that is an oxide of iron.  Furthermore, if iron is exposed outside in the weather for 100-year or more it would turn to rust and leach into the soil reverting back to a soft iron ore.

More specifically -

In metals we need to classify a metal as either a cathode or an anode.  A cathode is noble or protected and the anode is the opposite.  The anode is the metal that is corroded or gnawed away.  They must be in contact with each other either directly.  There must be a electrolyte or chemical solution (acid or alkaline).  Sometimes the electrolyte is moisture or soil, and sometimes that soil may also be mineral rich. 

In a desert, the sand is mineral poor and moisture deficient; therefore, the electrolyte is removed and corrosion is all most impossible to occur for carbon steel.  Just the opposite occurs in the tropical forest with rich moist mineral bearing soils in a humid environment where the electrolyte is present everywhere and coming in contact with the soil will cause corrosion of the carbon steel.

The electrolyte and chemical solution will have a pH.  With acid the solution is positively charged.  An alkaline the solution is negatively charged.  Remember a pH-7 is neutral.

Carbon steel is a slightly more alkaline than neutral and will not corrode in water that is slightly alkaline unless oxygen is present in the water and or when dissimilar metals are connected with the carbon steel like brass as an example.  

Oxidation of Carbon Steel Pipe in Water with Bronze Valve -

Pipe

Fe - 2e  → Fe⁺⁺ 

Two electrons left the carbon steel, Fe - 2e,  by being electrical connected or touching each other to a more attractive metal, bronze, wants the electrons based on galvanic reaction.  (note: this is why we like to use black iron pipe with black iron valves and fittings) What is left of the carbon steel is the iron with an unusual positive charge that has to leave the carbon steel and enters the electrolyte (water).

Water

Fe⁺⁺  + OH ^-  → Fe(OH)₂

The positively charged iron mixes with water, which will always strip the H₂0 apart to obtain the hydroxyl (OH ^-) ions (Note: The hydrogen ion will head over to the bronze).  The mixing occurs very-very close to the surface of the carbon steel and forms a ferrous hydroxide Fe(OH)₂.  In fact, this seems to occur at the surface and there is a lot of ferrous hydroxide Fe(OH)₂ forming, but this  ferrous hydroxide is unstable such that it will recombine in the water again and again until it becomes more electro-chemically stable.

Rust

Fe(OH)₂ + H₂O + O₂  → 4Fe(OH)₃

Ferric hydroxide 4Fe(OH)₃ is rust.  All rust is made this way in water and eventually the reaction becomes embedded in the carbon steel.

Once corrosion starts it can never to stopped 100%.

In closing, we can always expect corrosion, based on the galvanic reactions, between carbon steel and

• Magnesium (Magnesium will corrode)
• Zinc (Zinc will corrode)
• Aluminum  (Aluminum will corrode)
• Bronze  (carbon steel will corrode)
• Brass  (carbon steel will corrode)
• Copper  (carbon steel will corrode)
• Stainless Steels 300-series  (carbon steel will corrode)
• Titanium  (carbon steel will corrode)

There are many forms of corrosion.  Lets highlight a few, but remember they all are based on a chemical reaction of some sort -

• Stress corrosion is influenced by stresses in the metal like constant straining from pressure on a pipe or residual welding stress
• Fatigue corrosion is subjected to fluctuating stresses where the stress is removed or reversed then applied again
• High-temperature corrosion from sulfidation, de and carburization, and an oxide layer like mill scale
• Fretting corrosion is influenced by rubbing or parts that are fitted together that vibrate differently
• Intergranular corrosion is where the metal grain boundaries is chemical attacked.
• Impingement or Erosive corrosion that is subjected to either flow related condition in the presence of corrodent product or cavitation
• Leaching is a corrosion process where certain alloys are selectively removed from the metal

We hope that this overview helps you understand corrosion better.

Next - Quiz Introduction to Metallurgy

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