VOLUME 14 ISSUE 6
WHAT’S HAPPENING AT MISSOURI S&T (FORMERLY UMR):
SHORT COURSE DATES
We will be offering "Introduction to Paint Formulation" May 20-24 (Spring 2019). This course is intended to give the person a fundamental knowledge of how to approach a starting formulation and troubleshoot it. This course involves both lecture and laboratory work.
We are also offering a new course, "Coatings Composition and Properties for Sales and Marketing Personnel" March 25-27 (Spring 2019). This course is designed for those in the industry who buy and/or sell raw materials into the coatings market, as well as those who buy or sell coatings or simply use coatings. The course is intended to help the newer person in the field gain a better understanding of the science behind paint. For more information, including course times and fees, click the course name above.
For more information see our web site at http://coatings.mst.edu and to register contact us at mstformulation@mst.edu or call 573-341-4419. **These courses are held on the Rolla Campus**
ONLINE SHORT COURSE
We are offering "Introduction to the Coating Systems" online short course. This course is targeted for automotive and aviation type OEM companies. This self-paced seminar will cover the painting system from the composition of paints to the evaluation of the dry film. The pigments, resin, solvents and additives will be discussed including their influence on the coatings performance. Color measurement, surface profile, and other evaluation criteria will be related to composition. The importance of surface preparation and other manufacturing criteria will show the system complexity and each step's importance.
We are offering "Surface Defects: Elimination from Human and Process Contaminants" online short course. This course addresses many of the issues in prevention and minimization of defects. The course covers the defects caused by the coatings process, as well as human issues, including personal care product causes. Several of the surface defects are discussed – from basic principles and real world automotive and aircraft examples. The highly practical approach of this course will greatly aid the personnel involved in the painting operation to reduce and systematically approach issues.
EMPLOYMENT TAB
There are no current job openings available. Anyone wanting to have job opening listed, please contact us at (573) 341-4419 or e-mail: mstformulation@mst.edu . You can also write to us at Missouri S&T Coatings Institute, BOM #2, 651 W. 13th St., Rolla, MO 65409-1020. Our web site is http://coatings.mst.edu
TECHNICAL INSIGHTS ON COATINGS SCIENCE
An Overview of Salt Spray Corrosion Testing- Cyclic vs Noncyclic Methods
Abbie Braden, Graduate Research Assistant, Missouri University of science and Technology
Resistance to corrosion is an important property of coatings, especially those that will be often exposed to the outdoor elements such as industrial and automotive coatings. Corrosion occurs when oxygen and water are allowed to reach the surface of the substrate. This may be caused by defects in the coating film, or deterioration from weathering. Most coatings customers have a specified amount of exposure that they expect a coating to uphold to, and this must be tested and proven by the manufacturer. The test most manufacturers use for this is salt spray, or salt fog tests.
The American Society for Testing and Materials, or ASTM, defines many standard procedures for salt spray tests, depending on the type of coating being tested, or type of environment the coating will be exposed to. All these standards can be found on the ASTM website. The method that is most applicable for this topic is ASTM B117-16 Standard Practice for Operating Salt Spray (Fog) Apparatus2.
The standard mentioned above defines the way the salt spray cabinet must operate, including the temperature, concentration of salt (5% NaCl) in the solution, air supply, and collection rate of the spray/fog. It also describes sample preparation and the placement of samples in the cabinet. One thing that is rather curious about this standard is that it claims in section 11.1 “Unless otherwise specified… the test shall be continuous…” This means that during the entire testing period, the samples are being bombarded with a concentrated salt water solution. Unfortunately, this is not a very accurate representation of what the applied coating will actually be exposed to.
For this reason, many companies are now considering performing cyclic salt spray tests. ASTM has defined cyclic methods for acidified salt, SO2, and dilute electrolyte fog, which can be found in the appendices in Standard G85-114. The main difference in these four versions are the composition of the salt solution. The test procedure would depend upon the environment to which the coating will be exposed.
Another cyclic testing method involves alternating between salt spray and UV exposure, which is now a feature in many cabinets1. This method is referred to as prohesion. ASTM Standard D5894-16 gives a procedure for this method that involves one week in a UV chamber at 4 hours light exposure then 4 hours exposure to condensation. This is then followed by one week in salt spray in cycles of 1 hour fog, 1 hour dry. The standard also defines the temperature of both environments, properties of the UV light, and salt composition. It should be noted that the salt composition in this method, at 0.05% NaCl, is more dilute than the previously mentioned tests, but it also includes 0.35% ammonium sulfate.
There are several other cyclic methods that may be used as well. One of these, mentioned in “Principles and Prevention of Corrosion”5 involves 0.25 hours in salt fog, 1.25 hours drying at ambient conditions, and 22.5 hours in salt spray at specified conditions. This is repeated for as many days/hours as necessary to meet the customer’s desires. The same book also mentions spraying automotive coatings twice a week to represent environmental exposure.
None of these tests are extremely representative of true natural exposure. It would be extremely impractical however, to test a coating in a “real” situation because that would take years! The most important thing when doing accelerated laboratory tests such as this, is consistency. This may not give an environmental representation but does allow for a good comparison between coatings. Below is an image of a control panel on a salt spray cabinet; the different functions and variables can be set and adjusted depending on the specifications for the coating being examined.
References
- Ascott Analytical, Corrosion Testing
- ASTM Standard B117-16 Standard Practice for Operating Salt Spray (Fog) Apparatus
- ASTM Standard D5894-16 Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal
- ASTM Standard G85-11 Standard Practice for Modified Salt Spray (Fog) Testing
- D. A. Jones, Principles and Prevention of Corrosion, 28, 486:488 (1996)