Never AgAiN

of the inorganic zinc. However, the

consultant did not follow the procedures

contained within ASTM D6677 and even

developed his own rating system, disre-

garding the rating system contained

within the standard.

Two other consultants, one hired

by the general contractor and the other

hired by the fabricator, performed

adhesion tests using ASTM D4541, (a

quantitat ive adhesion test method),

and their results provided a completely

different picture regarding the adhesion

of the coating system. ASTM D4541

testing indicated that approximately

96% of the areas tested displayed satis-

factory adhesion.

Why such different results? These

two test methods differ significantly in

several ways. One test is quantitative,

and the other is qualitative and subjec-

tive. ASTM D6677 states that the test

method is subjective and qualitative and

“does not have any known correlation to

By Mike O’Brien

AdhesioN TesT MeThods – MAkiNg sure The resulTs sTick

A b o v e ASTM D4541 (Test Method E) showing 65% cohesive fai lure in IOZ primer, 20% glue failure, and 15% cohesive failure in red topcoat.

Adhesion testing is commonly

used during coating failure

investigations; however,

sometimes the wrong type

of adhesion test is performed, the results

are misinterpreted, or the procedures

and reporting requirements in the test

methods are disregarded. This article

addresses some common issues related to

ASTM D4541 and ASTM D6677 adhesion

testing and provides readers with a proper

understanding of differences between

these two test methods.

Different ADhesiOn tests – Different resultsSeveral years ago, a consultant, working

for a large inspection firm in the United

States, convinced state Department

of Tra nspor tat ion (DOT) person-

nel that the adhesion of an inorganic

zinc (IOZ) primer was inadequate on a

large bridge with over 400,000 square

feet (37,161.22m2) of steel. The consul-

tant recommended that a l l coatings

be removed and replaced on this the

bridge. This recommendation was based

primarily on qualitative and subjective

testing he conducted on portions of the

bridge, reportedly in accordance with

ASTM D6677.

According to the consultant’s

report, greater than 61% of ASTM D6677

adhesion tests conducted on the exterior,

and 65% of the tests on the interior box

girders displayed poor cohesion strength

The purpose of the procedures within

a standard is to ensure that different

individuals perform the test following the

same procedures and that they report the

results using the same criteria.

74 CoatingsPro g July 2012

other test methods (pull-off, tape, etc).”

Additionally, it states, “Other adhesion

test methods may be useful in obtaining

quantitative results (See D2197, D3359,

D4541, and D7234).”

Regardless of the explicit state-

ments contained within ASTM D6677,

the DOT’s consultant refused to accept

the results of more than 260 adhesion

tests performed by two other consultants

in accordance with ASTM D4541. As a

result of the DOT’s consultant’s allega-

tion that the coating system would likely

fail catastrophically if not removed, the

steel fabricator and the general contrac-

tor each spent a substantial sum of money

on consultants and legal fees.

Overview Of AstM D4541 & AstM D6677There are several ASTM test methods for

assessing the adhesion of liquid-applied

coatings to a substrate or to a previously

applied coating. These test methods are

as follows: (1) ASTM D3359, Standard

Method for Measuring Adhesion by

Tape Test, (2) ASTM D4541, Standard

Test Method for Pull Off Strength of

Coat ings Using Por table Adhesion

Testers, and (3) ASTM D6677, Standard

Test Method for Evaluating Adhesion by

Knife. (Note: ASTM D7234, Standard

Test Method for Pul l Off Adhesion

Strength of Coatings on Concrete Using

Portable Pull Off Adhesion Testers,

addresses adhesion testing of coatings

on a concrete substrate.)

This article only discusses ASTM

D4541 and ASTM D6677. The reader

is advised to obtain a copy of each

standard and to follow it when perform-

ing testing.

AstM D4541This quantitative test method involves

a procedure for evaluating the pull-off

strength of a coating system from metal

substrates using portable mechanical,

hydraulic, or pneumatic adhesion testers.

It maximizes tensi le stress. Fai lure

occurs at the weakest plane within the

system, which includes the substrate,

each coating layer applied, the adhesive,

and the test dolly.

A test fixture, commonly referred to

as a dolly, typically composed of alumi-

num, is attached to a properly prepared

and cleaned surface using an adhesive

compatible with the coating system, and

with a rated strength (in psi or MPa)

that exceeds the minimum test strength

required for the test. After the adhesive

cures sufficiently, the adhesion tester is

mounted onto the dolly and a mechani-

cal, hydraulic, or pneumatic device

attempts to remove the dolly from the

coated surface.

Each component or layer involved

in the test is designated according to an

alphabetic naming convention detailed in

the standard. The substrate is designated

as location “A,” the first coating layer as

“B,” the next coating layer as “C,” and so

on. The adhesive or glue used to affix the

dolly to the coated surface is referred to as

“Y,” and the letter “Z” designates the test

fixture or dolly.

Three distinct types of failures are

possible under ASTM D4541, includ-

ing cohesion failure (failure within one

distinct layer), adhesion failure (failure

between two distinct layers), and glue

fai lure. Under ASTM D4541, a glue

failure may occur between the glue and

the test f ixture, between the topcoat

and the glue, or within the glue itself.

A cohesion failure is identif ied by a

single letter, which indicates the layer

within which the failure occurred. For

example, a 100% cohesive failure within

the primer is reported as “B”- 100%. Two

letters identifying an adhesion failure,

(i.e., B/C), indicate the layers between

which the failure occurred. The letter “Y”

indicates a glue failure.

The three genera l ty pes of test

equipment cited in this standard are

based on the means used to perform

the pull (i.e., mechanical, hydraulic, or

pneumatic). Each piece of equipment

is designated within the standard as a

Method using the letters B, C, D, E, and

F. The Test Method (instrument) used

for the test significantly affects the test

result values. Tables showing results

on four paint systems using dif fer-

ent Methods are contained within the

standard.

T he tables accompa ny i ng t he

Above ASTM D4541 (Test Method E) showing 100% cohesive failure within the IOZ primer.

July 2012 g www.coatingspromag.com 75

standard show that tests conducted

u s i ng t he pne u m at ic e qu ipment

(Method D) and the hydraulic equip-

ment (Methods C, E, and F) consistently

achieve significantly higher test results

than the mechanical portable adhesion

tester (Method B). When reporting

results, ASTM D4541 requires report-

ing the instrument used to conduct the

test. The increase in test values using

the pneumatic or hydraulic testers,

compared with the mechanical adhesion

tester, is significant, often doubling the

value obtained using the mechanical

adhesion tester.

Other factors affecting test result

values include the following: (1) the

degree to which the surface is prepared

prior to attaching the dolly, (2) the

method by which the dolly is prepared

prior to affixing it to the surface, (3) the

adhesive selected, including the surface

temperature during the cure of the

adhesive and the time interval between

attaching the adhesive and performing

the pull.

Prior to attaching the dolly, the

coated surface is cleaned in a manner

that does not affect the coating’s integ-

rity and does not leave a residue. The

standard allows for light abrading of

the coating using 400-grit sandpaper

or finer, followed by solvent cleaning to

remove particulates from the abrading

process. The solvent selected must not

compromise the coating’s integrity.

Data published by DeFelsko on

its Website found that how a dolly is

prepared can result in a significant

difference in pull strength. The results

indicate that preparing the dolly using

Scotch-Brite pads resulted in an increased

adhesion test value of 837 psi (5.77 MPa)

compared to a machined dolly, and an

increased adhesion value of 180 psi (1.24

MPa) compared to using sandpaper to

abrade the dolly.

Ma ny d i f ferent ad hesives a re

used for ad her ing a dol ly to t he

coated surface. Generally, solventless,

two-component epoxies provide the

best results. Different types of epoxies

are available, including some fast-set

epoxies that reach their full strength

and can be tested within 1 hour after

application. The drawback for some

fast-setting epoxies is a reduction in

their ma ximum pul l test strength.

Slower curing epoxies generally require

a 24-hour cure time, but they also allow

for higher psi testing, some up to 5,000

psi. Selecting an epoxy that cures at the

anticipated surface temperature is criti-

cal. The time required between applying

the adhesive onto the dolly and perform-

ing the pull test is a function of surface

temperature and epoxy formulation.

For tests performed in accordance

with ASTM D4541, at least three tests are

required in the area tested to statistically

characterize the area. General reporting

requirements include the following: (1)

general nature of the test (field or lab), (2)

temperature and relative humidity during

the test period, (3) details regarding the

test apparatus used, and (4) a description

of each coat in the coating system.

Specific requirements related to the

test results include the following: (1) the

date, test location, testing location, (2)

the values, nature, and locations of the

failures (using the location designations

included in ASTM D4541), and (3) if

scoring around the dolly was performed.

Certain coatings types, such as

inorganic zinc (IOZ) primers, typically

fail cohesively. For example, in a three-

coat system consisting of IOZ primer,

epoxy intermediate coat, and polyure-

thane topcoat, the expected mode of

failure is cohesive within the zinc primer.

Therefore one should not be alarmed by

a cohesion failure within the IOZ as long

as the value at failure is at an acceptable

level. Consult the coating manufacturer

for its acceptance criteria.

AstM D6677This adhesion test method uses a knife to

determine the adhesion of a coating to a

substrate or to another coating. Using a

sharp knife, two cuts, each 1½" (38.1 mm)

in length, at a 300 to 450 angle, are made

in the coating film, forming an “X”. The

cuts must extend down to the substrate.

The point of the knife is placed at the

intersection of the cuts, and an attempt is

made to lift the coating from the substrate

or from other coating layers. Sufficient

tests must be performed to ensure the

adhesion evaluation is representative of

the entire structure.

Above Cohesion, adhesion, and glue failures illustrating proper location desig-nations in accordance with ASTM D4541.

76 CoatingsPro g July 2012

ASTM D6677 conta ins a sca le

with ratings from 0 to 10. A rating of

“10” indicates the coating is extremely

difficult to remove with fragments no

larger than approximately 1/32" by 1/32"

(0.8 by 0.8 mm) removed with great

difficulty. A rating of “0” indicates the

coating can be easily peeled from the

substrate to a length greater than ¼" (6.3

mm). Ratings of 2, 4, 6, and 8 contain

simi lar statements. The higher the

rating number, the more diff icult the

coating is to remove and the smaller the

paint chips size removed. In accordance

with ASTM D6677, any coating removed

during the cutting process is to be disre-

garded.

ASTM D6677 conta ins severa l

statements about the test method itself

and about its relat ionship to other

adhesion test methods. These include

the following: (1) this method is a quali-

tat ive and subjective test , (2) other

adhesion test methods may be useful in

obtaining quantitative results, and (3)

no known correlation exists between

this test method and other adhesion

test methods, such as pull-off (ASTM

D4541) or adhesion by tape (ASTM

D3359). ASTM D6677 contains no preci-

sion and bias statement due to this test

method’s subjective character.

Reporting requirements for ASTM

D6677 include the following: (1) number

of tests, (2) mean and range of the tests,

(3) location of the failure (between the

first coat and the substrate, between the

first and the second coat, or within a

coat), (4) structure tested, (5) location

and environmental conditions at the time

of the test.

ClOsinG reMArksASTM standards and other indus-

try standards are consensus standards

writ ten with procedures, report ing

requirements, and precision and bias

statements. The purpose of the proce-

dures within a standard is to ensure that

different individuals perform the test

following the same procedures and that

they report the results using the same

criteria. However, for some ASTM tests,

such as ASTM D6677, no precision and

bias statements are included since the test

is subjective in nature.

When an ASTM test method lists a

rating scale, those who perform the test

in accordance with the standard must

employ the rating scale when reporting

the results. Developing an independent

rating scale that differs from the scale

provided within the standard is not

permitted. When a forensic investiga-

tor chooses to blatantly disregard the

procedures and instructions contained

within an ASTM standard and/or devel-

ops his or her own rating system, which

benef its the cl ient ’s predetermined

position, he or she is acting irrespon-

sibly and not in accord with integrity

required for conducting independent,

fair investigations. cP

A b o v e Tests per formed in c lose proximity using different test methods as follows: Triangle (ASTM D6677), Circle (ASTM D3359), and Trapezoid (ASTM D4541).

July 2012 g www.coatingspromag.com 77