About Laboratory Testing
Testing methodology was developed and implemented to allow the screening of multiple coating systems for their suitability in high durability, non-immersive service, specifically in C5-M and C5-I highly corrosive environments per ISO 12944.
The methodology employs exposure of coating system samples for 1440 hours to either a conventional, neutral salt spray or a more aggressive acidified salt spray, with pre- and post-exposure evaluation of the coating system samples by tensile adhesion testing to determine changes in bond strength, and electrochemical impedance spectroscopy (EIS) to determine changes in coating barrier properties.
Testing Methodology
This testing methodology is based on ISO 12944-2:2017, Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Part 2: Classification of environments, corrosivity categories and durability ranges, and on ISO 12944-6:2018, Corrosion protection of steel structures by protective paint systems – Part 6: Laboratory performance test methods , laboratory performance test methods.
Testing also included electrochemical impedance spectroscopy (EIS) evaluation of each coating system before and after 1440 hours of exposure to salt spray and condensing humidity.
The maximum duration of the testing is 1440 hours, which corresponds to the coating’s ability to withstand a very highly corrosive industrial environment (C5-I, per ISO 12944-2:2017) or very highly corrosive marine (C5-M) environment, with high durability.
Corrosivity Categories
C1 – Very Low
No exterior evnironments
C2 – Low
Atmosphere with low level of pollution; rural areas
C3 – Medium
Urban and industrial atmospheres; moderate sulfur dioxide pollution; coastal areas with low salinity.
C4 – High
industrial areas and coastal areas with moderate salinity.
C5-I – Very High (industrial)
industrial areas with high humidity and aggressive atmosphere.
C5-M – Very High (Marine)
Coastal and offshore areas with high salinity.
Durability Categories
Low Durability
Less than 5 years to first major maintenance
Medium Durability
5 to 15 years to first major maintenance
High Durability
Greater than 15 years to first major maintenance
Coating System Laboratory Testing
The laboratory testing methodology for evaluation of the coating systems include the following:
Tensile adhesion testing per ASTM D4541-17, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers ; both as-received and after salt spray exposure testing.
Salt spray testing per ASTM B117-19, Standard Practice for Operating Salt Spray (Fog) Apparatus, for a period of 1440 hours. Samples are also evaluated for blistering, rusting, cracking, and flaking at each time interval. EIS analysis is conducted before exposure and at 1440 hours.
A separate set of scribed samples is evaluated for scribe creep-back after 1440 hours of salt spray exposure per ASTM D1654-08(2016)-e1, Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments27.
Condensing humidity testing per ASTM D2247-15(2020) , Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity, for a period of 1440 hours. Samples are also evaluated for blistering, rusting, cracking, and flaking at each time interval. Based on ISO 12944, time intervals for evaluation included 120 hours, 240 hours, 480 hours, 720 hours, and 1440 hours. EIS analysis is conducted before exposure and after 1440 hours.
Rating and Selection of Coating Systems
The laboratory testing protocol recommends eliminating coating systems for consideration based on key criteria defined as follows:
· ASTM D4541-17 tensile adhesion testing of less than 2000 pounds per square inch (13.79 MPa), either as-received or after salt spray exposure – Coating adhesion properties are critical to long-term service in corrosive environments, such as coastal/marine or heavy industrial.
· ASTM D1654-08(2016)-e1-procedure A scribe creep test rating of less than 8 (creepage greater than 1/32 inch or 0.8 millimeter) – Coating system must not significantly peel or blister under severe, corrosive service conditions if subjected to mechanical damage.
· Maximum impedance (in giga-ohms, GΩ); as determined by electrochemical impedance spectroscopy; after salt spray or humidity exposure greater than 1 GΩ (109 Ω) – Coating barrier properties must be maintained under long-term corrosive service conditions.
· Percentage of initial protection; as determined by electrochemical impedance spectroscopy; after salt spray or humidity exposure greater than 95% – The rationale for this criterion is the same as the preceding criterion with respect to coating barrier properties.
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