Presented by: Bob Kogler, Rampart LLC

Protecting Steel Edges

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-PRF-23236C

• Compares the film thickness of 3 specimens cut from a sample of coating applied to a non-chamfered 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)

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 URETHANEEDGE RETENTION

System:

Zinc Primer

Polyaspartic Urethane

System:

Zinc Primer

Polyaspartic Urethane

Edge Retention > 70%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