Magni Wind Power Article... - The Magni Group, Inc.

FASTENING, JOINING, & BOLTING
Choosing the right coating for fasteners
T
he importance of fastener coatings is
becoming more apparent throughout
the wind industry. Different coating
chemistries have been used to improve
corrosion protection, or lubricity, or both.
However, increasing demands on field life, global
legislation, and worldwide commonization
continue to create challenges for many
traditional coatings.
A thorough understanding of a fastener’s
coating performance can ensure joint integrity,
as well as improve field life and serviceability.
Global acceptance and availability are also key
considerations in choosing the right coating.
Advantages of fastener coatings
The importance of corrosion protection and
lubricity in the wind industry differs from
the building trades or the appliance industry. Varying
demands have made it impossible to formulate one
universal coating. A short list of what a coating should do
includes:
• Improve appearance
• Add lubricity
• Protect against corrosion
Appearance
This is of lesser concern in most turbine joints, but it
plays a key role when choosing a fastener coating in the
building trades and appliance industry. Electroplated
and electrocoated fasteners are predominant in these
industries. Because such fasteners are often visible,
it is essential to match colors and coat uniformly.
Electroplated fasteners offer good uniformity and come
in several different colors using different passivation
systems, dyes, and a topcoat. Electrocoated fasteners
also offer good uniformity but are typically available only
in black.
16 WInDPoWEr EnGInEErInG & DEVELoPmEnT APrIL 2012 www.windpowerengineering.com
Edward Koneczny
Windpower market manager
The magni Group, Inc
www.themagnigroup.com
The Magni 565 coating allows formulating lubricants into the topcoat,
usually eliminating the need for messy post-treatments. This
non-electrolytically applied, zinc-and-aluminum-rich coating also
eliminates concern of hydrogen embrittlement. The coating can be
formulated in many colors. It also comes in several friction levels
while providing good consistency across many bearing surfaces.
Lubricity
Identifying a coating with the right frictional characteristics is
probably the most crucial task. Erratic friction during assembly leads
to either a loose joint or too much strain on the fastener. Either
event can lead to a failed joint.
Two important components of lubricity are consistency and
efficiency. Both are required to insure joint integrity and easy
assembly. Proper lubricity assists the fasteners’ ability to efficiently
reach its designed load at a prescribed torque. The torque-tension
graphs (next page) demonstrate how properly integrated lubrication
can improve joint consistency.
Some alloy platings also have improved lubricity but are typically
more expensive than zinc plate or dip and spin coatings.

Input torque versus clamp force
for a Zinc+ lubricant-coated bolt
Zinc-coated bolts show high deviation in the torque required to
get several to a 30,000-n clamp load.
Corrosion protection
The harsh environments of wind farms
have significantly increased demands
on corrosion protection. The increasing
demand on field life, as well as the use
of aluminum and magnesium in other
assemblies, emphasizes the importance
of high-performance fastener finishes.
Bimetallic assemblies often rely on
coatings to reduce galvanic potential or
insulate different materials in an effort
to minimize corrosion rates. For this
purpose, a duplex coating is becoming
more prevalent.
500 hours NSS, 1000 hours NSS
The three bolts on the left have been
hot-dipped galvanized (HDG) and subjected
to ASTm B117 for 500 hours. The
three on the right, coated with magni
565, have endured the same test but for
1,000 hours.
Mechanics of corrosion protection
Although there is a myriad of finishes to
choose from, there are only two basic
types of corrosion protection: Barrier
protection and sacrificial protection
Both protect the substrate from
corrosion, but do so in distinctly different
ways. To better explain these methods of
protection, it is first important to review
the corrosion mechanism. Iron corrodes
by anodic, cathodic, and diffusion
reactions. The reactions are described
as:
Anodic reaction
Fe → Fe 2+ + 2 e-
Cathodic reaction
4 e- + 2H 2o + o 2 → 4(oH - )
Diffusion reaction
2Fe 2+ + 4(oH - ) → 2Fe(oH) 2
Fe(oH) 2 + o 2 + H 2o → Fe 2o3 + H 2o
Corrosion prevention inhibits the rate of
these reactions, with either a barrier or
sacrificial coating to the substrate.
Input torque versus clamp force
for Magni 565-coated bolts
Magni 565 coated bolts provide better uniformity in the torque
needed to produces a 30,000-n clamp load.
Barrier protection
one way to protect ferrous substrates
is with a barrier coating. Here, the
coating works as a barrier between a
corrosive media and the substrate. Such
coatings, often organic and polymeric
in nature, come in a wide array of colors
and can be applied many different
ways, including by electrophoresis
(electrocoat). These can be extremely
effective, but performance diminishes if
the barrier (coating) is damaged during
assembly. These defects, macroscopic
and microscopic, are inevitable and give
the environment access to the substrate.
once this happens, oxidation of iron,
along with the cathodic reduction of
dissolved oxygen, attributes to the
generation of oH¯ (alkalinity). Generated
alkalinity can react with the coating
causing it to dis-bond or delaminate
from the metal interface, ultimately
causing coating failures.
Barrier protection works best when
coating defects are kept to a minimum.
Excessive coating damage diminishes
the corrosion protection proportionally
and leads to premature failures.
Therefore, material handling is critical
when this technology is used.
www.windpowerengineering.com APrIL 2012 WInDPoWEr EnGInEErInG & DEVELoPmEnT 17

F A S T E N I N G , J O I N I N G , & B O L T I N G
Sacrificial protection
Another corrosion protection option
applies a sacrificial coating. These
include electroplating-galvanizing,
Hot Dip Galvanizing (HDG), along with
inorganic-organic duplex coatings.
Although the application method for
sacrificial coatings can vary significantly,
the protection mechanism is similar.
Sacrificial corrosion occurs when
two dissimilar metals come in contact.
The more active of the two metals in
the cell becomes a sacrificial anode
and cathodically protects the other. To
protect steel fasteners from corrosion,
they are often coated with metals of
higher reactivity. Zinc-rich coatings,
whether electroplated, galvanized,
or non-electrolytically applied, offer
cathodic protection for steel fasteners
by letting the coating “sacrifice” itself
in place of the ferrous fastener. This
technology relies on the coating to
preferentially corrode.
Sacrificial coatings are affected less
by coating defects than barrier coatings.
The coating, which contains a more
reactive metal, still provides cathodic
protection around small voids and
defects. However, excessive material
handling can diminish the performance
of this technology as well.
Understanding how these two
systems provide corrosion protection
makes it clear why duplex systems,
such as magni 565, can offer high
performance to conventional
galvanizing. The topcoat’s resistance of
a corrosive media’s coming in contact
with a sacrificial coating delays the
18 WInDPoWEr EnGInEErInG & DEVELoPmEnT APrIL 2012 www.windpowerengineering.com
corrosion mechanism. In addition, these
topcoats can be formulated to provide
specific frictional characteristics. WPE
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