Use and Care Manual

MiTek
®
Product Catalog
Copyright © 2020 MiTek Industries, Inc. All Rights Reserved
12
Corrosion Information
Relative Corrosion Resistance Capability:
The chart below ranks the available options in terms of their relative
effectiveness against corrosion. As expected, the ability to resist corrosion
increases with increasing zinc thickness, so G185 is the most durable
pre-galvanized product available. Gold Coat offers enhanced protection
compared to G185 while stainless steel offers the best protection for
most applications.
Galvanic Corrosion:
The simplest and most practical solution to minimize galvanic
corrosion is to make sure that the components that are in direct contact
with each other are made of the same material or coating. Once this is
achieved, there is no net galvanic potential between the components and
galvanic corrosion is eliminated or signicantly reduced. For example, use
galvanized nails for galvanized connectors and stainless steel nails
for stainless steel connectors.
Wet Service Condition:
For lumber, this refers to any service condition in which the average
equilibrium moisture content is 15% or more over a year or may exceed
19% at any time. For lumber to get above 19% moisture, the relative
humidity in the air needs to reach above 80%. Unfortunately, this is above
the critical humidity level for the electrochemical oxidation of steel, which
is around 70%. Beyond 70%, the rate of corrosion in the connectors
increases rapidly due to the abundant availability of moisture.
G90 may not be suitable for use in wet service condition.
Preservative (Pressure) Treated Wood:
There are many preservative wood treatment formulations available on
the market today. The element that is common to most of them is the
presence of copper in the formulation which can contribute to the
corrosion of steel connectors and fasteners.
Of the copper based preservatives, the two types are micronized copper
and soluble copper. Micronized copper formulations MCA (micronized copper
azole) and MCQ (micronized copper quat) are sold under different brand
names and are the most predominant formulation in today’s preservative
treated wood industry. Soluble copper formulations CA (copper azole) and
ACQ (alkaline copper quat) have also been very popular since they replaced
CCA (chromated copper arsenate) which was phased out in 2004. Some
“metal free” preservatives are still used for above ground and sill plate
applications, but are not as common. One of the main criterion affecting the
selection of one preservative treatment over another is the type of wood
being treated and how well it can be penetrated by the treatment.
While many of the advanced wood treatment formulations containing
copper used today have proven to be less corrosive to steel, especially
micronized copper, MiTek recommends a higher level of corrosion
protection for connectors in contact with copper based wood treatments.
Connectors and fasteners in contact with metal free wood preservatives
do not require additional corrosion protection due to the preservative itself,
however all factors that can create the corrosive environment should be
considered when selecting the appropriate nish. If unsure as to whether a
particular treatment is corrosive to steel fasteners, check with the supplier
of the preservative treated wood product for their recommendation.
Fire Retardant Treated (FRT) Wood:
Although most common FRT products are not corrosive to metal
connectors, not all products are non-corrosive. Additionally, they
typically require proprietary strength reductions applied to the lumber
in accordance with the manufacturer’s specications. Since the lumber
strength is lower, the lateral and withdrawal resistance of nails must also
be reduced accordingly. It is important to note that some re retardants
cause the wood to absorb more moisture from the air than untreated
lumber. Consequently, the connector may be exposed to a higher level
of moisture, resulting in more corrosion.
Swimming Pools:
This is one of the most hazardous environments for steel connectors due
to continuous exposure to high temperature, high moisture content, and
corrosive chemicals such as chlorine, bromine, and other disinfectants.
The combination of all these factors can lead to accelerated corrosion
and premature structural failure. This environment is so corrosive that all
possible preventive measures should be employed to prevent the hanger
from being exposed to the pool water. These include the use of a vapor
barrier and a ventilation system that does not take the air from the pool
environment.
Additionally, it has been known that certain grades of stainless steel
(316 and others) are susceptible to a mode of structural failure known
as stress corrosion cracking (SCC) when exposed to a swimming pool
environment. SCC is usually localized near areas of high residual stress
and small cracks can rapidly propagate and cause catastrophic failures.
See warning below.
Gold Coat may be the best choice in this environment.
WARNING
Stainless steel connectors and fasteners shall not be used for
metal hangers over swimming pools due to stress corrosion
cracking. SCC has been known to occur under the following
conditions:
• Use of certain grades of stainless steel (grades 316 and others).
• Structural members subjected to high tensile stress.
• Presence of certain chemicals, including chlorine and bromine.
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Relative Corrosion Resistance Capability:
G90 TZ HDG GC SS
(G185) (Gold Coat)