TO 35-1-3 - Robins Air Force Base

TO 35-1-3
1.6.3 Anode and Cathode Surface Area. The rate of galvanic
corrosion also depends on the size of the parts in contact.
If the surface area of the corroding metal (the anode) is
smaller than the surface area of the less active metal (the cathode),
corrosion will be rapid and severe. However, if the corroding
metal is larger than the less active metal, corrosion will
be slow and superficial. For example, an aluminum fastener in
contact with a relatively nonreactive Monel structure may corrode
severely, while a Monel bracket secured to a large aluminum
member would result in a relatively superficial attack on
the aluminum.
1.6.4 Climate. Warm, moist air, typically found in tropical
climates, tends to accelerate corrosion because of the higher
concentration of water vapor in the air. Additionally, high temperature
environments tend to produce more rapid corrosion
because of accelerated chemical reactions. Conversely, cold
dry air, typically found in colder regions of the world, tends to
slow corrosion rates due to less water vapor in the air. Corrosion
does not occur in very dry conditions. For this reason,
desiccants or dehumidification systems are placed in, or connected
to shipping or storage containers to create very dry
local environments. The equipment’s proposed operational
environments and established corrosion prevention and control
optimal design parameters should be considered in SE design
and procurement.
1.6.4.1 Desert. Hot, wind-swept deserts create a severe
maintenance problem because powdery dust can penetrate
even supposedly sealed components. Typically, desert conditions
are thought to have low humidity and rainfall. However,
there are areas of the world considered as desert with high
daytime temperatures, moderate to low night time temperatures,
high humidity (in areas such as the Persian Gulf), ultraviolet
radiation, and fine dust. These conditions pose the most
serious, destructive elements of the desert climate due to the
wide range of conditions experienced daily.
1.6.4.2 Temperate Zones. The temperate or intermediate
climate zone encompasses most of the North American and
European continents. These areas at various times of the year
may approximate the extremes of polar, desert, or tropical
temperatures and humidity. The temperate zone temperatures
range from -25 °F to +59 °F (-32 °C to +15 °C) in the winter
and from +59 °F to +125 °F (+15 °C to +52 °C) in the summer.
The relative humidity (RH) also fluctuates between five
and 100%. The most critical areas are coastal locations, during
the warm periods of the year in which the relative humidity
approaches 100% RH at night and the air has high concentrations
of salt. Moisture from this salt laden air can condense on
equipment during early evening and morning hours, thereby
causing serious corrosion. Because of its relatively mild temperatures,
the temperate zone is also the most heavily populated.
Consequently the smoke, smog, ozone, and corrosive
fumes associated with heavy industry are also found there.
1.6.4.3 Tropics. One of the greatest challenges to SE manufacturing
industries is to the design equipment that is protected
from corrosion and deterioration in the continual heat and
humidity of tropical climates. Even though they encompass
only a small portion of the earth's land area, the tropics
demand the greatest amount of consideration from the standpoint
of corrosion treatment and control. Relative humidity of
up to 100% RH at ambient air temperatures of 85 °F (29 °C)
and above create a formidable threat of corrosion. When high
humidity and temperature conditions are combined with saltladen
air, the corrosive environment becomes extremely
severe. The critical combination of high temperatures, condensation,
high relative humidity, and contaminants such as salt
and sand may cause premature structural failure of equipment
due to corrosion. Deterioration of the materials used in electronic
equipment may also be accelerated. Thin film corrosion
on electrical contact points and pins act as insulators to electrical
current adversely affecting the operations of SE.
1.6.4.3.1 Factors of Influence in Tropical Environments.
Tropical environments are noted for long periods of
heavy rainfall, during which 100 inches or more of rain may
fall. Extended periods of high heat and humidity contribute to
rapid corrosion of metals, cracking and flaking of rubber and
plastic materials, and deterioration of seals. Equipment,
whether stored or in use, requires special protective containers/measures
and frequent preventive maintenance. Microorganisms
multiply excessively in tropical environments and
attack many non-metallic materials. Many items become covered
with fungi in a matter of hours. Electronic equipment
requires special efforts for effective operation in the tropics.
Intensive preventive maintenance and the best possible protective
techniques are necessary for SE and their components in
tropical environments.
1.6.5 Heat Treatment and Grain Direction. When heattreated,
heavy sections of metals do not cool uniformly
because the metallic grain/crystalline structures tend to vary in
chemical composition from one part of the metal to another.
This can cause galvanic corrosion if the metal crystals are
more active than another or contain contaminates. Alloys that
are fabricated by rolling, extruding, forging, or pressing have
properties that depend highly on the metal grain/crystal direction
(parallel to grain elongation vs. cross grain). For example,
exposed end grain corrodes much more easily than flattened
elongated surfaces in sheet stock. This explains why exfoliation
occurs at the structural edges or next to countersunk fasteners
on certain types of alloys.
1.6.6 Electrolytes. Electrically conducting solutions are
easily formed on metallic surfaces when condensation, salt
spray, rain, or rinse water accumulate. Dirt, salt, and carbon
deposits from engine exhaust and engine exhaust gases can
combine with water on wet surfaces thereby increasing the
electrical conductivity of the electrolyte and the rate of corro-
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