[No.4/17] The functions of the corrosion inhibitor contained in the rust preventive oil (2)
Last time, I explained about the effect of the corrosion inhibitor which prevent corrosion occurrence by the corrosion inhibitor being adsorbed on the steel surface. Let me explain again this time what substances are specifically used as the corrosion inhibitor.
The names of the corrosion inhibitor used for the rust preventive oil are not disclosed in catalogs and websites etc. When picking up from the professional literatures, the names can be as per Table 2. Quite a lot of “Metal salt” are listed. Although it becomes rather professional description, the corrosion inhibitor described in the name with “Metal salt” is classified into the group of substances referred as “Organic acid salt”.
As you know well, “salt” is composed by “acid” and “alkaline”. Table salt (sodium chloride) is also “salt”, in which chloride ion and sodium ion are tied in the strong bonding. But, “organic acid salt” is only associated with much weaker binding when compared the case with table salt. The original efficacy of the rust preventive oil would be lost by cutting off the chemical bond because of the invaded substances when the others come into the rust preventive oil from the outside. That is the reason why “Do not use under the strong acid condition or strong alkaline condition” is mentioned in the manual etc.
There are so many types of corrosion inhibitor in the rust preventive oil. Among of them, the barium sulfonate and barium-soap are widely used because of their excellent corrosion inhibitability. But, the development of barium-free rust preventive oil is being promoted, since the use of barium has been regulated in view of the safety, particularly in Europe and U.S. However, it is the real fact that there is not any excellent corrosion inhibitor to replace the barium sulfonate and barium-soap. As a result, a number of the manufacturers of the rust preventive oil have been repeatedly struggling to develop the alternatives.
Table 2: Major corrosion inhibitor used in rust preventive oil
|Sulfonium salt||Alkylbenzenesulfonic acid metal saltPetroleumsulfonic acid metal salt|
|Metal soap||Oxidized wax metal saltHigher fatty acid metal salt|
|Ester groups||Polyhydric alcohol oleic acid esterLanolin fatty acid ester|
|Metals used mainly: Barium, Calcium, Zinc|