Cursory data obtained from corrosion monitoring activities over the last decade have indicated that the simple act of placing various types of structure over or around areas being monitored often show significant reductions in corrosion rates.  For the purposes of this paper these effects will be covered under the general term “sheltering”.

If such beneficial effects could be generally verified, the application of the principals of sheltering may offer a simple means of corrosion control.  In addition to the effects of sheltering, further beneficial effects may arise from the application of dehumidification to nominally closed shelters.  These effects could apply to both aircraft and AGE equipment and is becoming of increased importance for storage of pre-positioned assets.

A study is in progress to quantify the effects of various types of sheltering, and in some cases with dehumidification, on the corrosion of metals. This has involved the installation of both small corrosion test cards with metal coupons along with corrosion sensors.  Placement of the corrosion test cards was typically along the sides of shelters.  The small corrosion sensors were usually attached directly onto various pieces of AGE equipment.  In all cases, the same monitoring devices were installed outside of shelters for comparison.  Readings and/or coupon exchanges occurred about every 3 months.

Various types of sheltering have been studied.  These have included conventional, tent-like structures for AGE equipment and aircraft sunshades, aircraft hangars, storage containers, and indoor avionics backshop areas with dehumidification.  Geographic locations have included a wide range of corrosive severity/ESI from the very mild (Holloman AFB, NM) to moderate (Pease ANG) to very severe (ships at sea/hangar bays).

Data will be shown for these various examples.  There is a clear message from the results that even simple and open structures will provide a considerable degree of corrosion attenuation.  Typical ratios of outdoor (unsheltered) corrosion to values under shelters range from a minimum of 4:1 to >20:1.  There has also been a trend in the data for the greater benefits to be found in those environments where they are most needed; i.e. in the more severe environments.  In the mild environments, there was little apparent benefit in terms of corrosion reduction, but it is recognized that in many cases shelters may be justified to reduce effects of UV exposure on coatings along with human comfort.

A preliminary analysis will be presented of why sheltering is effective at corrosion reduction.  This appears to involve a substantial reduction in atmospheric chloride which is also in agreement with a new model for atmospheric corrosion.

With this said, recent findings are indicating that there may exist unique exceptions to what has been discussed. At least one prominent example will be shown along with a discussion of current understandings of why higher corrosion rates have been found under an aircraft shelter.