Use and Care Manual

MiTek
®
Product Catalog
Copyright © 2020 MiTek Industries, Inc. All Rights Reserved
11
Corrosion Information
For the majority of applications, metal hangers and connectors are used in
interior, above ground, dry service conditions. They are typically not being
exposed to corrosive environments which can signicantly reduce their
strength and longevity.
What is Corrosion?
Corrosion is the destructive degradation of steel due to its interaction
with the environment. Here the steel is the connector and the environment
is whatever the connector interacts with, namely wood and air. Each
environment may contain one or more corrodents (substances that cause
corrosion) acting independently or in combination to degrade the strength
of the connectors.
Electrochemical oxidation is the most common type of corrosion affecting
metal connectors. It is a process in which iron (Fe) reacts with oxygen (O
2
)
in the presence of an electrolyte such as water (H
2
O) to form iron oxide
(Fe
2
O
3
), a brown and aky by-product commonly known as rust.
Steel is an iron-based metal alloy which is susceptible to this type of
corrosion, even when exposed to normal atmospheric air, since air
contains oxygen and water as part of its normal composition. While steel
is very strong, rust is not. Over time, the continuous formation of rust
eats away the base metal and reduces the strength of the connector.
The rate of oxidation generally increases with increasing moisture content,
the presence of salt, or when galvanic corrosion is a contributing factor.
Galvanic corrosion occurs when there is an interaction between
dissimilar metals that are in contact with one another. The degree of
corrosion depends on where the metals reside in the galvanic series, which
is a compilation of known metals and their relative reactivity. The more
active metal (anode) will corrode preferentially while shielding the more
passive metal (cathode) from further degradation. For example, with
galvanized steel, zinc is used as a coating on the steel because it
sacricially corrodes to protect the steel substrate underneath. The
coupling between zinc and steel is said to have a lower galvanic potential
than the coupling between zinc and stainless steel because zinc and steel
are closer to each other in the galvanic series. In general, the coupling
with a lower galvanic potential would result in a slower corrosion rate.
Corrosion Protection Options
Zinc Galvanizing:
Most connectors are manufactured from pre-galvanized sheet steel or
coiled steel, which is typically made by the hot-dip process in accordance
with ASTM-A653 and ASTM-A924 standards. Fasteners are galvanized
in accordance with ASTM-A153. In the manufacturing of the connectors,
the punching and shearing processes create exposed bare metal surfaces.
Thankfully, zinc has an incredible ability to ‘heal’ itself; the zinc around the
exposed metal corrodes and deposits a layer of zinc corrosion by-product
called zinc patina (white powdery appearance) over the exposed metal to
further protect it.
By being more reactive than steel, zinc sacricially corrodes at a steady
rate over time to shield the steel from the effect of corrosion. The protection
ability of zinc is proportional to its thickness, which is proportional to the
amount of zinc applied. Zinc coating is specied as the total weight on both
sides of the sheet steel, measured in ounces per square foot (oz/ft2). For
example, G90 means that there are 0.90 oz/ft2; G185 has 1.85 oz/ft2 and
would last about two times longer than G90. G90 is the minimum protection
for connectors and is standard in MiTek connectors.
Design Guidelines:
Where there are governing national or local building code requirements,
they should be used in the selection of the connectors and their protection
against corrosion. In the absence of such requirements, the decision rests
on the experience and judgment of the building designer/engineer. Design
guidelines are presented in this section to aid the building designer/engineer
in this selection process, but it is the responsibility of the building designer/
engineer to determine the most viable solution based on an evaluation of the
connectors to the specic corrosive environment(s). The guidelines consist
of best practices, recommended protection levels for the connectors, and
strength modication factors for the lumber/connectors.
Where there are multiple options suggested, do not automatically default
to the lowest protection level. The lower protection level is intended to
address less severe conditions while the higher protection level is meant
to address more severe conditions. Select the option that eliminates or
adequately reduces the vulnerability of the connectors to the corrodents.
When in doubt, use a higher level of protection than anticipated or seek
professional consultation.
METAL
Before After Healing
METAL
Wood Environment Air Environment
MOISTURE
SALT
CHLORINE
AMMONIA
HYDROGEN-SULFIDE
OTHER CHEMICALS
EXPOSED METAL
ZINC ZINC
ZINC PATINA
O
2
O
2
O
2
O
2
Iron + Oxygen Rust
Water
(Electrolyte)
Galvanic Series (Abbreviated)
More Active (Anodic-)
More Passive (Cathodic +)
Zinc
Aluminum
Steel
Brass
Copper Nickel
Stainless Steel - Type 304
Stainless Steel - Type 316
Continued on next page
MOISTURE
WOOD
PRESERVATIVES
FIRE RETARDANT