Examples of Pitting Corrosion Incidents

A survey was performed of incidents and accidents of aircraft and helicopters caused by pitting corrosion, where an incident is any damage to the aircraft and/or injuries to passengers and crew and an accident is loss of the aircraft and/or fatal injuries to passengers and crew. Data were taken from the NTSB and FAA web-sites, which include their databases of all aircraft incidents and accidents since 1983. It was determined that of the 91 incidents and accidents found under corrosion, seven of those gave the cause of failure as pitting corrosion.

Unfortunately, it has been found that there are problems in reporting the causes of the incidents and accidents in the NTSB and FAA in that the real cause of an incident or accident is not reported properly and, therefore, does not show up in the database. For example, in reading through some of the incidents and accidents caused by corrosion, it was found in the text that the real cause of failure was, more specifically, due to pitting corrosion or exfoliation. But those words were not highlighted so that incident or accident did not show up in a search of pitting corrosion or exfoliation.

This makes the validity of numbers of incidents and accidents caused by pitting corrosion questionable due to the fact that additional incidents and accidents may be listed under different causes and/or more general causes. Also included in the survey were the three Embraer 120 incidents involving propeller blades, the Aero Commander 680 lower spar cap, and the F-18 trailing edge flap failure. These civilian and military incidents and accidents were all due to pitting corrosion as shown in Table 1. When these examples are taken with the general information cited in the previous references they clearly show that corrosion related degradation is a significant safety issue in the assurance of structural integrity of aircraft.

Therefore, the potential regrettable occurrence of accidents from corrosion related crack nucleation is a constant threat to aircraft safety. The following quote from the recent NATO RTO conference on fatigue in the presence of corrosion adds some understanding to the need for greater effort to understand the potential role of effects of corrosion on structural integrity. Another quote from a different reference also sheds further light on this issue (36-page 1-1).

Thus, the community clearly now recognizes the potential impact of corrosion related degradation on structural integrity of aircraft. The need to understand the potential for the occurrence of corrosion on aircraft components is critical. Thus, to even begin the assessment of this potential the community needs to know the following.

In corrosion fatigue conditions, several studies showed greater increase in fatigue crack growth rates compared to "baseline" fatigue conditions. Although major efforts were expended to understand the crack propagation behavior of materials, a few studies have focused on the crack nucleation stage in the overall fatigue process [39-41]. McAdam first suggested that corrosion induced pits might act as stress concentrators from which cracks could form [42]. A large number of chemical or electrochemical factors such as potential, passive film, pH, and composition of environment are found to affect the pitting corrosion fatigue process. As well, mechanical factors such as stress range, frequency, stress ratio (R), and load waveform and metallurgical factors such as material composition, microstructure, heat treatment, and orientation can influence pitting corrosion fatigue process. Nucleation of cracks from corrosion pits were observed by many researchers including the works of Hoeppner [37-39], Goto [43] in heat-treated carbon steel, and Muller [44] in several steels. As well, in NaCl environment, lowering of the fatigue life due to the generation of pits in carbon steel [45] and 7075-T6 aluminum alloy [46] was observed under corrosion fatigue conditions.

Once the pit forms, the rate of pit growth is dependent mainly on the material, local solution conditions and the state of stress. Cracks have been observed to form from pits under cyclic loading conditions. Therefore, to estimate the total corrosion fatigue life of an alloy, it is of great importance to develop some realistic models to establish the relationship between pit propagation rate and the state of stress. Furthermore, pitting corrosion in conjunction with externally applied mechanical stresses, for example, cyclic stresses, has been shown to severely affect the integrity of the oxide film as well as the fatigue life of a metal or an alloy. Therefore, to understand this phenomena, some models based on pitting corrosion fatigue mechanisms have been proposed as discussed below.

Review of Pitting Corrosion Fatigue Models, D.W. Hoeppner, V. Chandrasekaran, and A.M.H. Taylor, University of Utah and FASIDE International Inc.