BEYOND
BY PETER CLEAVELAND
Today’s Coatings Provide Solid Protection for Valves
Like anything made of metal, valves are subject to corrosion and other
natural forces that compromise their
performance. However, the right coating can protect those valves not just
against corrosion, but erosion and wear.
Many coatings in use today, including
both organic (paints, for example) and
inorganic (zinc, chromium, nickel, aluminum and others) are applied to the
COURTESY OF DELORO S TELLITE
Figure 1. High velocity oxygen fuel coating of
a Ball Valve component.
outside surfaces to protect those surfaces. However, protecting the inside of a
valve is a bit trickier because of the myriad of factors that come into play. This
article concentrates on those challenges,
covering coatings for the insides of valves
that resist corrosion, prevent wear of the
trim and other moving parts, and reduce
erosion by abrasive media.
CHOICE OF METHODS
AND MATERIALS
A wide selection of coatings is available
today. Control Valves, part of the ISA Practical Guides for Measurement and Control
(1998), lists electroless nickel to protect steel bodies and bonnets from corrosion, aluminizing for protection from
high-temperature corrosion, boronizing
to prevent internal erosion and a variety
of sprayed coatings.
Many methods covered in this area
are presented by the companies that
make or provide them as alternatives to
chrome plating—which is desirable
because the newer coatings promise better properties and because chromium is
facing ever-stricter environmental
restrictions.
The requirements of individual applications determine which type of material is optimum for helping a valve resist
corrosion, wear and erosion, says Dore
Rosenblum, director of marketing, Sub-One Technology.
“For example, let’s say a valve is
operating in a purely corrosive environment—gases flowing or something.
Then, the goal is to make sure the coating is thick enough and doesn’t have any
holes in it or pinholes that can cause
corrosion,” Rosenblum explains.
The coating in this case could be a
simpler one than a valve that had a very
heavy sand flow, “where you have to
worry quite a bit about erosion or abra-
Electroless Nickel (EN) is generally an alloy of 88% to
99% nickel with the balance being phosphorous, boron or a
few other possible elements depending on the specific
requirements of an application. It can be applied to numerous metals, alloys and nonconductors with outstanding uniformity of coating thickness to complex geometries. In addition, its super fine particles can be added to form composite
EN coatings that can enhance existing characteristics and
add entirely new properties. Particles from a few nanometers up to about 50 microns in size can be incorporated into
coatings that are a few microns up to many mils in thickness.
The particles can comprise from 10% to over 40% of the
coating depending on the particle size and application.
The widest use in the valve industry for these coatings is
for increased wear resistance. Particles of many hard mate-
rials can be used in this process, such as diamond, silicon car-
bide, aluminum oxide, tungsten carbide, boron carbide and
others. Diamond is the most common and has a Taber wear
index of 1.159. Where low coefficient of friction, dry lubrica-
tion and repellency of water, oil and/or other liquids are cru-
cial, the coating can contain 20% to 25% of sub-micron
PTFE particles. Tests show that the lowest coefficient of fric-
tion is achievable when both mating parts are coated with
composite EN-PTFE. Where high heat is a problem, compos-
ites with particles of ceramics such as boron nitride (which
can withstand temperatures above 1562º F [850º C]) are
used. When loading is high, the coefficient of friction of EN-
BN and conventional EN actually decreases as the load
increases.
EN coating can also be made to fluoresce under ultraviolet light, which can be useful in telling genuine OEM parts
from counterfeit parts as well as in identifying specific manufacturing lots. The fluorescent material can even serve as an
indicator layer, warning when the coating has worn off and
replacement or recoating is necessary.
COMPOSITE ELECTROLESS NICKEL COATINGS BYMICHAELD. FELDSTEIN
MICHAEL D. FELDSTEIN is president of Surface Technology, Inc. Reach
him at Michael@surfacetechnology.com.
