Testing Corrosion Properties in Gasoline Containing DCI-6A

[jnoeland]

At our refinery, we are using NACE Standard testing method TM0172-2001 to test corrosion properties of our gasoline. We use corrosion inhibitor DCI-6A. We are seeing random test failures that do not appear to be correlated with relative amounts of any blending component or gasoline grade. We take a sample in a glass jar and pour out of the glass jar into the testing apparatus, is it possible that the corrosion inhibitor is coating the sampling jar and thus is present in lower concentration for the test? Is there other obvious troubleshooting strategies or things to look at?

 

[kenvlach]

I'm not familar with testing gasoline, but your hypothesis of the inhibitor forming a film on the sampling vessel may well be correct.

I've done environmental sampling (groundwater, waste acids & plating chemicals, etc.). The EPA sampling protoccol is to triple rinse the sampling container with the sampled fluid before taking the final sample to be tested. A waste container is used for the rinseate so as to not contaminate the sampled fluid.
Of course, sampling containers must be completely clean. If re-using the glass jar, are you cleaning between samples to prevent carry-over?

 

[jnoeland]

Yes the jars are cleaned between samples. Do you know what EPA protocol number or standard you are referring to by chance?

 

[kenvlach]

It's been a while -- might have to revive an old computer with a site assessment document containing a EPA-approved sampling plan. However, it's a general procedure. From
Standard Methods for the Examination of Water and Wastewater 16th Edn. (1985) pp. 37-44, "105. COLLECTION AND PRESERVATION OF SAMPLES," mentions general precautions.

"Before filling, rinse sample bottle out two or three times with the water being collected..."

"Before collecting samples from distribution systems, flush lines sufficiently to insure that the sample is representative of the supply, taking into account the diameter and length of the pipe to be flushed and the velocity of flow."

You can test for absorption of the inhibitor on the sampling vessel walls. Refill a vessel 3 times with a known inhibited gasoline, then thoroughly drain (upside down to apparent dryness. Next, fill the vessel once only with non-inhibited gas, cap it and shake a bit to re-dissolve inhibitor. Analyze for inhibitor content.

To be more scientific & quantitative, do the above test 3 times (with different jars), at low, medium and high inhibitor concentrations.
Plot the re-dissolved inhibitor analyses vs. initial inhibitor concentration. If the plotted line goes through (0,0), you can make a simple correction for your regular samples. If not, then there will be 2 factors; as a non-zero y (x = 0) indicates some inhibitor remains on the vessel wall even when exposed to virgin gasoline.
For 1st case, inhibitor is physisorbed only.
For 2nd case, some inhibitor also chemisorbs on the walls.
Ken

 

[jnoeland]

I'll give it a shot and see how it goes. I appreciate the help.