Rust and Blister Rating (ASTM D610 and D714)

ASTM D610 and ASTM D714 are visual rating methods used to describe the deterioration of coated surfaces. ASTM D610, “Standard Practice for Evaluating Degree of Rusting on Painted Steel Surfaces,” rates how much of the surface shows rust. ASTM D714, “Standard Test Method for Evaluating Degree of Blistering of Paints,” rates blister size and frequency. Both are used after laboratory exposure tests such as salt spray and cyclic corrosion testing and during field coating condition assessment.

ASTM D610: degree of rusting

Rust grades

D610 assigns a rust grade from 10 (no rusting, or less than 0.01% of the surface) to 0 (more than 50% of the surface rusted). Intermediate grades correspond to increasing percentages of rusted area. The scale is logarithmic in effect, so the small-percentage grades are more finely divided. Use the table in the current standard for exact boundaries.

Rust distribution

The standard also describes the pattern of rust with a letter:

• S (spot rusting): rust concentrated in a few localized areas.
• G (general rusting): small rust spots scattered over the surface.
• P (pinpoint rusting): very small, individual rust spots, as at holidays or pinholes.
• H (hybrid): a combination of patterns.

A full rating looks like “7-G,” meaning grade 7 with general distribution.

Reference images

The standard includes or references SSPC-VIS 2 color photographs for each grade and distribution. Inspectors compare the surface with the images under good lighting.

What counts as rust

Only rust breaking through or under the coating is counted. Rust staining that has run onto the surface from elsewhere is excluded, as are surface stains that do not originate at the substrate (see bleeding and staining).

ASTM D714: degree of blistering

Blister size

Size is rated on a numerical scale from 10 to 0:

• 10 means no blistering.
• 8 is the smallest blister easily seen by the unaided eye.
• 6, 4 and 2 are progressively larger blisters.

Blister frequency

Frequency is described with letters:

• F: few
• M: medium
• MD: medium dense
• D: dense

A rating such as “No. 6 M” means medium-size blisters at medium frequency. Reference photographs show each combination.

For more on causes, see coating blistering and osmotic blistering.

ISO 4628 equivalents

The ISO 4628 series covers similar evaluations with a different scale:

• ISO 4628-2: blistering, rated by size and density on 0 to 5 scales.
• ISO 4628-3: rusting, rated Ri 0 to Ri 5.
• ISO 4628-4: cracking.
• ISO 4628-5: flaking.
• ISO 4628-6 and -7: chalking.
• ISO 4628-8: delamination and corrosion around a scribe.
• ISO 4628-10: filiform corrosion (see filiform corrosion).

ASTM and ISO ratings are not directly interchangeable. Specify which system applies.

Related ASTM rating methods

• ASTM D1654: evaluation of painted specimens subjected to corrosive environments, especially creepage from a scribe. See undercutting corrosion.
• ASTM D4214: chalking.
• ASTM D661: cracking.
• ASTM D772: flaking.

Using the ratings

Laboratory panels

After a defined exposure, panels are rated for rust, blisters, scribe creep and other changes. Ratings let different coatings or surface preparations be compared under the same conditions. A panel result is a relative comparison, not a prediction of service life.

Field surveys

On structures, inspectors rate representative areas and record photographs. Ratings help owners rank assets for maintenance, choose between spot repair, overcoat or full removal, and track change over time (see coating maintenance planning).

Specifications

Some contracts use D610 grades to trigger maintenance or to define acceptance after a warranty inspection. When this is done, the specification should name the grade, distribution, area of evaluation and the method of selecting areas.

Practical tips

• Rate under consistent, good lighting. Avoid low-angle glare.
• Clean dirt from the surface gently before rating, without removing rust.
• Use the current printed reference images; photographs on screens may not reproduce color accurately.
• Rate representative areas and note worst-case areas separately.
• Record ratings alongside DFT and adhesion data to help diagnose causes.

In practice

In the field, an inspector usually rates rust and blistering together during a condition survey, recording the D610 grade and distribution and the D714 size and frequency for each area. Photographs and a simple map of the structure make later comparison easier. Owners often use these ratings to decide between spot repair, overcoating and full removal. Laboratory programs use the same scales to compare panels after salt fog, humidity or cyclic testing, which lets results from different labs and different years be read on one common basis.

Key terms

• Rust grade (D610). A scale from 10 (no rust) to 0 (essentially all surface rusted), linked to percentage of surface rusted.
• Rust distribution. Spot (S), general (G), pinpoint (P) or hybrid (H) rust patterns.
• Blister size (D714). Numbers from 10 (no blisters) down to 2 (large blisters).
• Blister frequency. Few (F), medium (M), medium dense (MD) or dense (D).
• Reference photographs. The pictorial standards that allow consistent ratings by different inspectors.

Frequently asked questions

Is rust grade 10 required on new work?

On new work, rust should not be present at all. Rust grades are used for evaluating aged or tested coatings.

Can blister size be measured with a ruler?

D714 is a comparison method using reference photographs, not a ruler measurement. Very large blisters may be described with additional notes.

Do these methods tell me why the coating failed?

No. They describe condition. Root cause requires investigation (see coating failure investigation).

Related entries

• Coating condition assessment
• Coating blistering
• Salt spray and cyclic corrosion testing
• Undercutting corrosion
• Filiform corrosion (new)
• Humidity and condensation testing (new)