January 2011 Archives

We have discussed so far on metal corrosion types, causes, and countermeasures. From this point on, let us look at some corrosion prevention measures.

The corrosion prevention basics are: (1) Use of corrosion resistant metals, and (2) Corrosive environment improvements. Details are as follows. (1) Make the metals electro-chemically more noble by alloying and/or plating and coating. (2) Convert corrosive environments into non-corrosive environments by adding soil neutralizers for objects embedded in corrosive soils, and add inhibitors (corrosion inhibiting chemicals) for water submerged objects. Additionally as related to (1), take the corrosion issues in considerations for the designing and constructions processes and eliminate crevice corrosions, concentration differential cell phenomena, uneven fluid flow, and localized heating. Electro-chemical Corrosion protection measures are widely applied for large ships, Bay/Port structures, and chemical plant equipment. Typically called electrolytic Corrosion protection measures, there are Cathodic and Anodic methods.

(1) Cathodic Corrosion protection Method

Ship hulls constantly being submerged in corrosive sea water suffers corrosions due to the hull steel being electrochemically non-noble where a cell is formed between copper alloy screws and hull steel material. Customarily, zinc ingots have been placed in stern areas to protect the hull corrosions. The zinc ingots were used as sacrificial metal where the zinc dissolves instead of the hull. The principle is the same as zinc plated roof plates. The zinc in this case is called Sacrificial Anode. There are two cathodic Corrosion protect methods. One method is to use an external DC source to polarize the metal to be protected as a cathode, and the other is to connect less noble metals. The former is called External Power Supply Method or Powered Method, and the latter case the zinc ingot method mentioned above is called Galvanic Anode Method or Sacrificial Anode Method. 1) For the sacrificial anode methods, zinc, magnesium, and aluminum can be used. However, aluminum cannot be used in fresh water since likely to become more noble than steel in fresh water, though can be used as non-noble metal in sea water since the passive surface layer will be destroyed. 2) For the external power supply methods, direct current, sufficient to overcome the corrosion current flowing out from the metals to the environment, is used. As anodes, non-dissolving materials such as graphite and ferrite.

Corrosions indirectly caused by metabolic byproducts of microbes is called "Microbiological Corrosions". The microbes do not consume the metals as nutrition sources causing the corrosions, but the metabolic byproducts and/or the germs themselves form corrosion cells causing anodic/cathodic reactions. The corrosion causing microbes are mainly bacterium, and sometimes mold fungus. Examples are iron-oxidizing bacteria that cause corrosions on steel, and steel/copper corrosions caused by sulfate-reducing bacteria. Additionally, aluminum alloy corrosions caused by mold fungus and bacteria in aircraft fuel are known. Classifications are shown below.

(1) Corrosion by Aerobic Bacteria

Aerobic bacteria require oxygen for growth and propagation, with the iron-oxidizing bacteria and sulfur bacteria being well known. The iron-oxidizing bacteria grows by absorbing the energy generated when oxidizing divalent iron ions (Fe2+) into trivalent iron ions (Fe3+) using water dissolved oxygen. This causes formation of rust bamps(tubercle) inside steel water pipes resulting in pipe blockages. The sulfur-oxidizing bacteria grows by utilizing the energy generated when oxidizing the remainder of incompletely oxidized sulfur, and eventually forms sulfuric acid. It can survive in strongly acidic environments of pH2~4.

(2) Corrosion by Anaerobic Bacteria

Anaerobic Bacteria does not require oxygen for growth, rather cannot survive in environments with oxygen. It was originally discovered in soil with little oxygen. Well known of anaerobic bacteria are sulfate-reducing bacteria, nitrate-reducing bacteria, and Methan producing bacteria. Sulfate-reducing bacteria reduces sulfates down to H₂S. The H₂S then oxidizes Fe and creates FeS. Contaminated metropolitan sewage water is low in dissolved oxygen due to decomposed organic compounds as well as contains large amounts of sulfate, providing ideal habitats for sulfate-reducing bacteria. The old contaminated Sumida river in Tokyo used to emit a stench during the summer, and caused blackening of copper alloys. The bacterial corrosions do not occur with AL, Zn, Pb and their alloys, and limited with Cu, but steel alloys are severely affected. The nitrate-reducing bacteria reduces nitrates and generates ammonia. The Methan producing bacteria create Fe(OH) 2 and CH4 (methane gas) from Fe and CO2.

(3) Microbiological Corrosion of Aluminum Alloys

Microbes living in water mixed in jet fuel and dissolved inorganic salts are know to cause problems such as Jet aircraft wing corrosions.

As countermeasures to prevent microbiological corrosions, it is essential to make the environment inhabitable to the microbes. In applications for aerobic bacteria the air is eliminated, and for anaerobic bacteria air circulation is provided.