Having Problems Testing a Corrosion Resistant Coating?

Testing a corrosion resistant coating can be challenging and confusing.  There seem to be an abundance of companies who use data to pull their punches.  Here are some ideas on how to navigate through the noise & evaluate the best coating for your application.

Just about everything will degrade or corrode over time given the right conditions.  So evaluating a coating for corrosion resistance can be more about how to properly evaluate the surface to yield useful data rather than exposing a surface to conditions so extreme that few materials can survive.  So if you're testing a surface for salt spray exposure at ambient temperatures, it does no good to design a test that exposes the surface to 150°C hydrochloric acid.  Either the coating will fail too quickly to yield any useful accelerated data, the data won't be relevant because HCl is not salt water, or you'll find a material or coating that will survive the test but cost way too much to be realistically used in the application.  In short, it's not about finding the toughest test, it's about finding the right test for the application.  That's where doing your homework comes in handy and learning that you should take vendor supplied corrosion data with a grain of... well...salt.  (pardon the pun.....)  So let's go through our to do list point-by-point.

Check the vendor data

• Check the vendor supplied coating data and see if it relates to your industry or application.

• Check the test data to be sure they're actually testing a coated part or just pulling material test data from a text book.  

Vendor corrosion data can tell you a lot about the company.  If you're in oil and gas, do they have sulfuric acid exposure data?  If you're in semiconductor fab do they have HBr data?  If you're looking for a coating that will stand up to salt spray, does the company offer salt spray data?  You get the idea.  If the company has some relevant test data, they may have some experience in your application and may be a better long term choice than a coating that has never been evaluated under the application conditions.  Also check the data to see if it compares performance over a wide range of materials. 

The data below compares corrosion rate in mills per year for several materials and the test coating, Dursan®.  The test is per ASTM G31 24 hour immersion of hydrochloric acid at ambient temperature.  Sometimes comparing the test coupons after testing can give you a general feel for how the coating will perform in the application.

Now don't expect a glove fit for the data.  Some companies may test at ambient temperature when your application is at 50°C or the concentration may not be the same.  That's where digging deeper into the data and requirements pays off.   Specifying test conditions that are similar to your application is helpful.  When designed properly, accelerated corrosion testing can significantly improve test turn around time. 

Also check the source of vendor data.  If the test was done per qualified lab and done according to an approved method you're on sound footing.  If the source of the data is actually from a materials handbook and not from a test lab, beware.  I've seen data that looked familiar only to realize the corrosion graph was pulled from a corrosion handbook that tested the bulk material not a coating.  So what's the difference?  Well let's use silicon as an example.  Silicon is completely inert and non-soluble in many acids.  So take a chunk of silicon and throw it in a beaker of acid and it will not corrode.  Great!  I can apply a silicon coating to stainless steel and have zero acid corrosion!  Nope, there's more to a coating than just the material itself.  The coating quality, coating density/pinhole , coating thickness and bond to the surface can all play a role in overall coating durability.  For example a coating may have a higher pinhole rate which can negatively impact corrosion resistance.  Never use bulk material data to pick a coating.  Paint doesn't rust so cars should never rust, yet they do!

Watch our corrosion video and see how we compare in acid immersion testing.

Digging deeper, do your homework.

• Go to your industry trade organization and see if they have test guidelines
• Go to NACE and check material and approved test methods
• Go to ASTM and purchase test methods

Your industry trade organization may offer some answers to your test questions.  Technical trade organizations like NACE can be very helpful in offering approved test methods.  Why go with industry approved methods?  Because an approved method will level the playing field so you're properly comparing the performance of various coatings or materials under the same conditions.  It seems like a simple idea, but no vendor data will match conditions with other vendors, so it's up to you to be sure comparisons are relevant and the same.

If you're serious about evaluating a coating, you should specify either a NACE or ASTM coating test method. Not doing so will provide too much wiggle room for a vendor comparison.  Like a through and complete set of architectural drawings and specifications will help in building the house you want, so will a complete set of test specs assure you get the test results you need.

The example below highlights some basic test results that include test conditions, test method, performance graphs and a visual comparison of results.  What's missing?  Multiple samples. 

Corrosion testing can be expensive, so there's always the temptation to test only one sample.  You'll learn a lot about coating variability by testing multiple samples.  Shoot for a minimum of 3 to 5 samples to get a better range of how a coating will perform.  The example below shows how multiple samples can show variability of a coating.  In this 5 sample ASTM 72 hour bleach immersion test, the Dursan® samples performed within a narrow range while the competing coating showed quite a bit of variability.

Read and understand the NACE or ASTM method then go to a lab

• Contract with an approved test lab.  You can waste a lot of money getting garbage data so you must do your homework regarding the recommended test method.

Once you know what test you want, specify that test to an industry approved corrosion test lab.  You can consult with the lab regarding test details but don't go to a lab thinking you'll ask them to recommend tests to evaluate a coating.  What you'll get is a virtual buffet of tests many of which can be frighteningly expensive.  You may get the results you need or you may get overkill or you may get garbage data (i.e. too aggressive a test for the application or a test not aggressive enough).  There's no short cut in corrosion testing.  Know your application, learn about the tests, then go to a lab with a test plan.  If you want to become a corrosion expert go to the Corrosion Doctors website.  They're all about teaching the novice about corrosion.  If you can't get enough corrosion like I can't get enough football, become a NACE Certified Corrosion Inspector!

Of course there are some labs that you can go to who do just the right amount of tests and offer relevant test results, just like there are contractors out there who will build a house with only a basic idea of what you want and you get a great job.  But you're playing the odds if you do your homework first.