by Juan Manuel Alvarez*
From the beginning of time the application of paints was given an aesthetic valuation and for many people today, it still has it. Increasingly aggressive environments due to industrial pollution and the increase in ambient temperatures, as well as extreme changes such as high temperature and high rainfall seasons, require high chemical resistance from coatings so that they have a good aging.
The above relates the aesthetics that we want to observe so much in the coatings that protect those surfaces, which from the economic point of view, are the assets that in some structures are supremely expensive, as is the case of aircraft.
In the aeronautical industry the specifications of the application of paint and coatings are given through manuals given by the aircraft manufacturer such as SPM (Standard Practice Manual) SRM (Structural Repair Manual) and CPCP (Corrosion Prevention and Control Program).
In the SPM (Manual of Standard Practices) there is a chapter in which the specifications are given based on which product quality standard must be worked, for example:
a)MIL-R-81294, Remover of epoxy and polyurethane coatings on non-ferrous metal surfaces.
b)MIL-C-5541, Passivator for aluminum alloy surfaces, especially for 2xxx and 7xxx series.
c)MIL-P-23377, As a specification for the First or Epoxy primer for non-ferrous metal surfaces, such as aluminum alloys, particularly 2024-T3 and 7075-T6 which are those found on exterior and interior surfaces when a greater service is performed or when the coating is completely changed, performing complete removal with the MIL-R-81294 standard product.
d)MIL-83286, High solid polyurethane with high resistance to abrasion and ultraviolet rays recommended as a finish by Bombardier, for example.e)MIL-T-81772, thinner diluent type I for epoxies and type II for polyurethanes.
Also, times such as induction (reaction), application (pot life), working temperatures, humidity range of the environment, air velocity, viscosity of the mixture, wet film thickness and dry film thickness, drying times for second layer, for taping, for decals and jigs, for full curing or flight availability for external surfaces of the aircraft; are recommendations or specifications that appear in this manual, in a chronologically ordered way and in a technical dialect typical of the specialty of paints and coatings.
The SRM (Structural Repair Manual) cites the specifications that must be met related to the types of repairs that are executed and the area of the aircraft being repaired, the above because depending on the area of the aircraft, the specification may change.
For example, the dry film thickness of the Primer or primer changes if it is applied to exterior surfaces or for interior surfaces, the finish or product of resistance to the environment for exterior surfaces is polyurethane, while for interior surfaces film inhibitors are applied which will also change depending on which surfaces are applied (floors, electrical terminals, joints, etc.). There are cases in which this manual directly gives the specifications in terms of procedure such as times, temperatures, humidity, viscosities and other variables, and cases in which it refers to the compliance of specifications by procedure to the SPM (Manual of Standard Practices).
Corrosion control
The purpose of the CPCP (Corrosion Control and Prevention Program) is to maintain the level of protection that coatings provide to surfaces, in such a way that it recommends the periods of cleaning, monitoring, inspection, replacement, relating these periods with the types of environments or aggressiveness of the environments (peaceful – rural and urban, moderate – warm and humid, aggressive – industrial and maritime) through which the aircraft transits or spends the night.
This means that although this manual is suggested by the manufacturer of the aircraft and delivered with it, the program to be followed will be the one that the operator or user of the aircraft adjusts or adapts to the operating and environmental conditions through which it fulfills its routes or itineraries. The cleaning and washing of the aircraft is such an extremely important task that depending on how it is carried out, with what products and the end of the surface decontamination is verified, the problems or reports due to corrosion will decrease in a representative way and will have greater importance depending on the operating conditions and environment through which it transits.
The monitoring that is carried out for the condition of the coating due to defectology is a practice that must be understood as preventive, the inspections that are carried out to the coating such as the dry film thickness, porosity and adhesion are techniques that must be applied under the parameters of the program as predictive maintenance. The parameters that are affected and that merit a replacement of the coating system, will be valued in the program as corrective maintenance. All information relating to work related to coatings must be recorded and carried by aircraft, in such a way that the result of the analyses at the different levels of maintenance and their periodicity, will give a vision of the protection of the surfaces and will result in the aging of the aircraft and its surfaces.
Corrosion costs in the aeronautical industry are very high, particularly when these tasks and procedures are not valued from the point of view of corrosion control but merely as an aesthetic result, a concept through which it is undervalued and where many times even the bad practices of these tasks induce or generate corrosion problems. and therefore they are part of the problem and not the solution.
It is very pleasant to highlight the change that has been made in Colombia, particularly in the last 5 years, regarding the importance that has been given to coatings, since today the aforementioned tasks are being duly carried out such as paint application procedures, maintenance of coatings and cleaning of aircraft. Positioning themselves as corrosion control methods that certainly are and that have already shown positive results in terms of the objective for which they are intended.
Through the training and advisory work carried out, the empirical work of these tasks has been disappearing. It is required to certify personnel to be technical paint applicators who also perform defectology assessments and periodic inspections, and not painters who only perform corrective maintenance work. Similarly, due to the looming future of aviation in Colombia, more certified coating inspectors are needed to maintain the level that corresponds to the practices of coating application and maintenance, as well as certified corrosion personnel who perform and demand strict compliance with inspection procedures, corrosion treatment and control.
* A.S.T.M. Diagnosis and Prevention of Corrosion, (NACE International, the Corrosion Society). [email protected]
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