Despite the best efforts, the corrosion control of mission-critical equipment’s components using inhibitors can fail. And when this happens, more often than not the root cause is oversights in adhering to best practice in the application of inhibitors.
There are no guarantees in equipment maintenance. One of them is the flawed assumption that an intervention will always work out as expected, or desired, just because it did in a recent project at some industrial site in Africa.
Controlling corrosion, where an inhibitor has been specified as a suitable method, is not an exception to this phenomenon.
Typically, inhibitors can be deployed in different ways. Mainly, these are: used in water treatment, impregnated into plastic wrapping, placed inside a solvent, oil or water, a grease, paste, impregnated into paper, soaked up into a sponge, or allowed to sublimate from a solid into a vapour that saturates the air or other gases in which a component is placed.
Unfortunately, a highly rated inhibitor can fail to stop the progress of corrosion as expected using the preferred of any of these methods. This could frustrate reliability and maintenance managers.
So, why would a tried and tested corrosion inhibitor fail?
“There is no cure-all in fighting corrosion. Much as we wish there were one to simplify the task,” observes Greg Combrink in response. Past President of the Corrosion Institute of Southern Africa (CorriSA) and founding director of Total Corrosion Control, he has been on sites across Africa helping heavy industries control corrosion for years, where he has noticed familiar trends in projects.
Combrink is eager to share his experience on corrosion inhibition: “If applied properly, an inhibitor can have a significant level of success. However, in most cases where inhibitors fail to perform, the root cause could be wrong product selection.”
Adherence to best practices
To address the challenge, he stresses the importance of adhering to best practices, suggesting the following critical steps:
1. Understanding How Inhibitors Work
It is important to understand how inhibitors work. Typically, inhibitors are chemical compounds or materials that protect metals in two main ways, explains Combrink:
“The first way is by interfering with the chemical reactions that occur during the corrosion process.
The second way is by being absorbed onto surfaces. This increases the resistance of the corrosion system by slowing down charge transfer on the interfacial double layer between the substrate and the environment.
This is typically achieved by forming a ‘passive’ monomolecular layer on cathodic or anodic regions of the surface, which significantly slows the rate of corrosion.”
Another factor to bear in mind is that specific inhibitors work better on one metal as opposed to another. That is why, to cater for this aspect, in a typical product brand range, most developers include a blend of different types termed ‘multi-metal’ inhibitors. These are designed to protect different metal types from corrosion within the same system.
That is why determining the right inhibitor for a specific environment or metal should start with testing.
2. Accurate and Relevant Corrosion Testing
Usually, a standard test involving exposure to salt fog is used. Typically, it is ASTM B117, which has been in use for nearly 100 years.
Nonetheless, standardised tests may not always apply in every situation, as Combrink illustrates: “What’s the point of testing metal in acid when, in reality, it will be exposed to an alkaline environment? The main problem is that people test without understanding the nature of the corrosion, and just blindly believe the results they obtain.”
In addition, compounding matters is that often corrosion occurs simultaneously via different mechanisms. Due to this phenomenon, the actual initial cause can be different from the subsequent mechanism that predominates. So, eventually, what starts as one form may evolve into another.
Hence, if this change is not recognised, the test may produce wrong data, which may affect subsequent processes.
3. Correct Application of Corrosion Inhibitors
Once testing ascertains the nature of corrosion, proper inhibitor use is paramount. In this area, Combrink points out, two aspects are essential: avoiding under-dosing and understanding inhibitor behaviour.
• Avoiding Under-dosing
To avoid under-dosing, there are two cardinal rules.
The first one is: never skimp on the active inhibitor. This is because using too low a concentration compromises performance and results, Combrink points out. “There is a critical concentration below which inhibitors lose effectiveness. Even if a slight reduction in corrosion is observed, the effect is misleading.”
Secondly, corrosion involves both the metal and the environment. Thus, the dosage must be based on both, and always exceed the critical threshold.
• Understanding Inhibitor Behaviour
Inhibitors can be complex. If an analyst is not aware of how they work, the wrong test method or misinterpretation of results may occur. For this reason, understanding inhibitor behaviour is vital.
4. Proper Sampling Techniques
While there could be a basic understanding of the inhibitor required, sampling errors can compromise the integrity of the entire analysis. Hence, Combrink highlights two common mistakes that must be avoided as far as possible.
a. Manhandling the Sample
Firstly, handling samples with bare hands can introduce contaminants like oils or food residue. “Salt and vinegar from your lunch could contaminate a specimen and skew results. You may even damage delicate corrosion products, obscuring evidence,” Combrink advises.
b. Cutting the Samples
Cutting a sample introduces friction and heat. Potentially, this destroys evidence even if a coolant is used. Consequently, sampling must be carefully planned, representative, and clean.
Generally, there should be no compromise on adherence to the above-mentioned aspects in sample preparation. This explains why in high-precision labs, dirty or mishandled samples may be rejected, especially for tools like Scanning Electron Microscopes (SEM), which require uncontaminated specimens.
5. Awareness of Common Procedural Errors
While the sample to be tested may be representative, several procedural errors can invalidate results. Particularly, the following cases are common:
• Salt Fog Chamber Setup
Large chambers may have uneven salt fog distribution. One side may have different exposure conditions from the other. This situation can compromise result comparisons.
• Calibration Test Pieces
In the chamber, there are supposed to be homogenous conditions. Unfortunately, running calibration pieces alongside actual samples may interfere and introduce significant variability.
To illustrate this challenge, Combrink asks: “How can you compare test specimens if different parts of the chamber expose them to different conditions?”
6. Matching Test Methods to Real Conditions
Once everything is in place, the next step is matching test methods to real conditions.
Ideally, everyone prefers a representative test method that produces guaranteed desired outcomes in analysis. However, this does not always apply in corrosion inhibition, clarifies Combrink, emphasising that there is no one-size-fits-all approach.
“In corrosion, it is horses for courses. There are standard test methods that are available, such as ASM and ASTM, and car manufacturers’ processes and ISO, and many more. However, at all times a representative test method must be determined to match the actual test conditions so that the test results have actual meaning.”
Otherwise, selecting the wrong method, even if it is widely accepted, could produce wrong results.
7. Critical Steps During Analysis
Last but not least, Combrink underlines three critical steps to ensure sound analysis:
• Critical Thinking
As part of critical analysis, he explains: “You must understand the corrosion process, the influencing factors, and what you’re looking at. And don’t let anyone contaminate your sample.”
• Statistical Analysis
Use multiple samples (three or more) to ensure reliability. This is especially important in legal contexts, where unreliable results can lead to costly losses in court.
• Equipment Calibration
Also, all machines should be in good working order and calibrated to an acceptable standard, preferably traceable to a national standard.
Proactive, Cost-effective Corrosion Management
Evidently, corrosion inhibition often fails not because of product shortcomings. More often than not, this could be due to gaps in understanding inhibitors, poor testing, improper application, and overlooked basics like sampling and equipment calibration. By following the above-mentioned, this can be addressed.
