![[ Visit Client Website ]](images/banner.gif)
In the case of repair of aluminium parts of military aircraft, conversion coating treatments are generally used for passivation, surface preparation and corrosion protection. In particular, as widely used, the chromating treatment has to be mentioned. Though the method is highly effective it is actually no longer used in the German armed forces. It can not be conclusively ruled if the acidic strongly oxidizing solution with an pH-value lower than 4 may cause increasing corrosion in gaps of the aircraft structure. Acid conversion treatment solution, once penetrated in capillary gaps between adjacent parts cannot be removed by standard flushing and cleaning as it is when applied to free surfaces. In this case detrimental corrosive attack may result.
To circumvent this problem in the German Armed Forces it was decided to stop the use of acidic conversion treatments as part of repair. Alternative measures are looked for. In addition the general replacement of chromates (VI) in corrosion protection systems are forced by health and environmental regulations.
Since for some time Cr (III)-based conversion coatings are offered, the topic of possible corrosive attack in capillary gaps is again under examination.
Laboratory experiments demonstrated that entrapped chromic acid in capillary gaps caused no corrosive attack. Thus, it could be demonstrated that the conversion treatment used for years by many nations has no disadvantage in the gap. But a complete change to the new environmental friendly treatments with Cr (III) also seems not to be possible. The investigated conversion solution on Cr (III)-based evoked significant pitting corrosion in the samples tested.
To use environmentally friendly conversion solutions on complex aircraft structures, the treatments must still be optimized in terms of their suitability for non-rinsable areas of the airframe.