Transcript Protecting Steel Edges Presented by: Bob Kogler, Rampart LLC Learning Outcomes • At the end of this webinar, you will be able to: –

Protecting Steel Edges
Presented by: Bob Kogler, Rampart LLC
Learning Outcomes
• At the end of this webinar, you will be able
to:
– Recognize sharp edges, outside corners,
crevices and welds
– Define the proper steps to be taken to treat
sharp edges, outside corners, crevices and
welds in order to avoid reduced coating barrier
protection
Introduction
• This webinar will describe:
– The importance of edge protection to the overall
corrosion protection of the structure
– The reasons coatings tend to often fail at edges
– the advantages and disadvantages of specific
methods of obtaining sufficient coating thickness on
steel edges and other irregular steel surfaces, where
coatings tend to pull away during curing, leaving thin
or non-existent coverage
What is an “edge”?
•
•
•
•
•
Corners (inside/outside)
Cutouts
Welds
Fastener heads
Anything with an angle approaching 90
degrees (or worse)
Why do we can particularly about edges?
• Sharp edges, outside corners, crevices
and welds are often sites where coating
failures and corrosion begin due to
reduced coating barrier protection
Reduced coating thickness is a result of surface tension
created during the curing of the wet coating causing the
coating to shrink, pulling the coating film away from the
edges
- solventborne coatings
Typical Failure initiation points
Avoiding Edge Failure
• Design out the edges and irregularities
Avoiding Edge Failure
• Fabricate out the edge problem
Avoiding Edge Failure
• Surface prep to remove the failure point
Avoiding Edge Failure
• Coating selection to mitigate the failure
point
Each Takes Investment
• Its all about tolerance for risk
– How harsh is the exposure –
locally at the edge?
– How long does the coating
need to perform?
– Will there be access for
inspection and maintenance?
– Consequences of a coating
failure?
Steps for Avoiding Reduced Coating
Barrier Protection
• The following steps are explained in SSPC PA
Guide 11, Protecting Edges, Crevices, and
Irregular Steel Surfaces by Stripe Coating
– Step 1: Eliminate the irregularities by grinding,
mechanical sanding or filing
– Step 2: Apply a thicker protective coating film to
problem areas (Stripe Coating)
– Step 3: Apply a full coat of paint with edge retention
properties, allowing application of a penetrating
and/or good wetting coating to problem areas
Step 1
• Outside corners and sharp edges should
be:
– Ground such that the edge is rounded to a
1/16-1/8 inch radius (NSRP SP-3 Panel
reports)
– Chamfered, which reduces a sharp 90 degree
corner to two 135 degree angles
• The distance between the two 135 degree angles is
approximately 1/16-1/8 inch
Step 1 (Continued)
Step 1 (Continued)
• The increased performance resulting from
rounding or chamfering edges depends on:
– The service environment
– The generic type of coating
– The radius or size of the chamfer
– Proper coating application per SSPC-PA 1,
Shop, Field, and Maintenance Painting of Steel
Step 1 (Continued)
• Note: Research has shown that
rounding or chamfering had no
measurable effect if an ethyl silicate
inorganic zinc-rich (IOZ) coating is
applied. Additional information on
this topic can be found in, Corbett,
W. D. “The Same Old Grind...An
Investigation of Zinc-Rich Primer
Performance Over Steel Corners.”
Modern Steel Construction, Vol. 40,
No. 6: p. 43-49
Step 1 (Continued)
• For best practice, edge grinding should be
done in the shop, BEFORE blast cleaning
where the work is accessible
Step 2
• A “Stripe Coat” is a coat of paint applied either
before or after a full coat is applied to an entire
surface. A “Stripe Coat” is typically applied to:
•Edges
•Welds
•Outside corners
•Bolt heads and threads
•Crevices
Step 2 (Continued)
• The project specification should clearly state
stripe coating requirements:
– The areas that are to be striped
– How to apply the stripe coat (brush or spray)
– In what order to apply the stripe coat (before or after
the full coat)
– Which coats are to be striped
– Whether tinting of the stripe coat or use of another
color of coating is required
– Details on the drying/curing requirements between
application of the stripe and full coat
Step 2 (Continued)
•Situations where stripe coating is
usually warranted are:
•Immersion service (e.g., interiors
of fuel or water storage tanks; ship
hulls)
•Areas with high corrosion rates
(e.g., cooling towers and splash
zones)
•Areas where access is difficult
(e.g., towers, some bridges)
•Built up members (e.g., lattice
bars and boxes)
Step 2 (Continued)
Examples of Where Strip Coating Is
Warranted
Step 2 (Continued)
• Situations where stripe coating may not be
cost effective are: (RISK?)
– Non-critical components (e.g., interior
bulkheads in habitable space on a ship)
– Mild exposure (e.g., SSPC Environmental
zones 1A and 1B)
– Inside corners (can be adequately covered
using proper spray technique)
Step 2 (Continued)
• Application of the stripe coat can be performed by brush
or spray. The table below shows the advantages and
disadvantages of the 2 methods
• Brush – control and working coating into
complex details; labor intensive
• Spray – faster, less labor, good for edge
retentive coatings and zinc-rich; may
bridge small defects, risk of high builds
Step 2 (Continued)
• When applying a stripe coat:
– The brush strokes should run parallel to, not across,
edges and crevices
– Apply constant pressure of the brush during application
– Bolt heads and nuts should be striped in a circular
motion
– Extend the stripe coating at least 1 inch from edges
and other irregular surfaces
– Use round or oval brushes
Step 2 (Continued)
• Spraying the stripe coat can reduce the amount of
time spent in brush application, but the painter has less
control over the precise boundaries of the area being
coated
• The applicator should spray the coating along the flat
surface, not directly at the edge itself, to prevent the
atomization pressure from pushing the coating away
from the edge
Step 2 (Continued)
• It is good practice for the stripe coat to be a
different color from the adjoining full coats or
substrate
– Enables painters and inspectors to visually
verify there are no misses, skips or thin spots
Step 2 (Continued)
• -When to Stripe- Before or After????
Step 2 (Continued)
• Welds and crevices are best protected when
striped with a coating with good wetting properties
• High build coatings are best for striping edges
• Consult coating manufacturer to determine the
appropriate coating to be used
• The coating selected should be designed for the
service environment
Step 3
• An alternative method for protection of
edges that may avoid the need for grinding
and chamfering is the use of a coating that
has edge retention properties
• Typically high solids products that provide
improved film thickness retention at the
apex of an edge
Step 3 (Continued)
• A method for measuring the edge retention
properties of a coating is published in MIL-PRF23236C
• Compares the film thickness of 3 specimens cut
from a sample of coating applied to a nonchamfered 90 degree angle
• The ratio of the film thickness at the apex vs. the
film thickness on the flat area is calculated and
expressed as the percentage of edge retention of
the coating
Step 3 (Continued)
he gauge and thin
uracy of the gauge
ulted to determine
g. If the stripe coat
ed over a surface
ganic zinc primer
nc coatings do not
c zinc. High solids
Measure paint thickness here (dft edge)
Measure paint thickness
here (dft at)
E
Measure paint thickness
here (dft at)
PA
SU INT
BS
TR
AT
ected when striped
s. High build coatr, it is impractical
a job with edges,
f a consistency to
SSPC-PA Guide 11
August 1, 2008
PA
I
N
T
Figure 3. Diagram of edge retention test
(courtesy of MIL-PRF-23236C)
The edge retention percentage is calculated as:
% Retention = dft (edge)/dft (flat) x 100
Step 3 (Continued)
• According to MIL-PRF-23236C:
– Any specimen having less than 50% edge
retention fails the test
– An average value of 70% edge retention for 3
test specimens measuring using an optical
microscope is required to qualify a coating
POLYASPARTIC URETHANE
EDGE RETENTION
System:
Zinc Primer
Polyaspartic Urethane
Edge Retention > 70%
Summary
•Edges and complex geometries are the key to
success or failure in a typical coating job
•SSPC Guide 11 is a provides a nice roadmap to
Good Painting Practice for addressing edges
•By following the processes described in Guide 11
you can provide extra corrosion protection measures
on edges, outside corners, crevices, bolt heads,
welds and other irregular steel surfaces
Summary (con’t)
• Avoiding edge failure in a PRACTICAL and
ACHIEVABLE manner requires a
specification that is clear and targets added
effort and material (cost) toward the details
that are most likely to fail first
• Recent success has been achieved in
mitigating risk of edge failure through the
use of new, high solids, rapid cure coating
systems IN CONJUNCTION WITH
implementation of improved practices
The industry continues to look for value based
solutions
Optimizing QA