Identifying and Solving MIC

Corrosion-resistant alloys (CRA) are used for almost every equipment, machinery, or apparatus used in the food, dairy, and beverage manufacturing. Despite that, the maintenance costs due to corrosion in these kinds of equipment are surprisingly very high.

Current research in corrosion science and engineering shows that microbes or bacteria have been causing corrosion problems in several other industries as well; from aviation fuels, to oilfields and pipelines, to nuclear waste. In the food, dairy, and beverage manufacturing industries, the same problem exists due to this microbiological issue.

Microbiological induced corrosion (MIC) on plant equipment can be identified by understanding the mechanism behind the development and propagation of MIC. By identifying MIC, its damaging effects can be minimized, if not eliminated. The accurate identification of MIC problems and applying preventive maintenance solutions can effectively reduce overall maintenance costs. Two separate plants experience scenarios that demonstrate how

Scenario 1: Sanitary Mixproof Valves

A plant manager of a dairy products factory was experiencing problems with sanitary mixproof valves, which were part of the clean-in-place (CIP) system. This caused disruption in production, contributing to unplanned downtime and product loss.

The company hired CSI to repair all defective valves. Upon thorough inspection of each of the valves, CSI experts found several problems: failed snap rings, broken and leaking air supply lines, and broken actuator springs.

CIS experts were able to determine the specific cause of each of problem, such as incompatible gasket materials, moisture in the air lines causing springs to corrode and fail, and misapplication of heat on the valves (which caused the gaskets to melt and the snap rings to break).

The failing gaskets in the mix proof valves was the most significant problem. The failing gaskets caused food and CIP fluids to become trapped. Microbes, or bacteria, began to build up on the machined surfaces of the valve stems. The residue from the bacteria induced serious pitting and corrosion, which led to the costly solution of needing to buy or rebuild parts to replace the damaged ones.

These photos show the failed gaskets on the valve parts and the MIC and pitting on the parts.

These photos above show MIC and pitting on the valve parts.

Without having a systematic preventive maintenance program, the problems with the gaskets were discovered too late. Their failure, due to incompatibility with the process material, caused food and CIP fluids to enter and become trapped in the areas around the gaskets, which led to the development MIC.

As a result, CSI experts recommended a comprehensive preventive maintenance program, with a focus on a proactive approach against MIC for all sanitary mixproof valves. The maintenance program included the following:

1. Changing all valve gasket material that is compatible with the process material.
2. Creating a preventive maintenance schedule to replace existing gaskets with ones that are compatible with the product. This included regular schedules for visual inspections, pullout of sample valves for thorough internal inspections, and major rebuilds.
3. Developing a cleanliness map to help Quality Control, Engineering, and Maintenance monitor specific areas for issues.
4. Conducting and documenting a risk assessment of each valve and how it affects product quality, safety, or legal compliance.
5. Testing air supply lines for contamination and cleaning.
6. Cleaning the control panel valves and replacement of the defective ones.
7. Establishing a replacement plan for the valves that are reaching the end of their useful service life.

After implementing CSI's recommendations, the client reported an annual return on investment (ROI) of US $70,000.

Scenario 2: CIP Return Piping

A plant maintenance manager of a cheese factory called CSI to investigate the leaks from corrosion that regularly appeared in the CIP return line. The company sought to understand why a traditionally used corrosion-resistant 304L stainless steel pipe would leak due to corrosion. They had doubts regarding the quality of 304L material and believed the material was possibly not 304L.

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This sample shows three areas in the pipe that have through-the-wall leaks.

The client provided a sample piece of the 304L stainless steel tubing, where the leaks were observed. CSI conducted several laboratory tests on the sample to check for material integrity. The tests included photomicrographs, wavelength dispersive X-ray fluorescence (XRF-WD) spectroscopy, optical emission spectroscopy (OES), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) on the inside and outside surfaces of the sample item.

The results of the spectroscopy tests showed that the elemental composition of the sample conforms to that of a typical 304L stainless steel material. It also revealed unremarkable, but significant, amounts of other elements that appeared to be contaminants and small amounts of phosphorus and chloride, which were most likely from CIP process chemicals.

Consequently, sulfur (S) was found on the pits on the inside surface. Sulfur is believed to be an indication of some sulfate reducing bacteria. The brown material on the inside surface was verified to be iron oxide, or rust.

The spectroscopy results on the pits on the inside surface show the occurrence of sulfur, which is believed to be an indication of sulfate reducing bacteria.

Photomicrographs from a metallographic examination indicated the sample to have the microstructure typical of a 304L stainless steel material. Furthermore, it revealed that the shape of the pits were consistent with microbiological-induced chloride pitting.

The photo micrographs show the morphology of the pit. The pit is broad and large. The shape of the pit indicates acidic conditions in the pit. The morphology is consistent with microbiological induced chloride pitting.

From the laboratory results, CSI experts were able to identify the cause of corrosion on the CIP return pipe as MIC. The bacteria, and resulting corrosion, could be caused by stagnant conditions in the pipes. Had the pipe been operating in full volume, it would have been impossible to form corrosion due to MIC.

CSI was able to prove that the company’s CIP chemicals used, such as the 1% caustic, could not have been the cause of corrosion in the pipes. CSI's findings very strongly indicated the presence of stagnant residual water or residual CIP chemicals at the lower drain regions of the system, which was likely the cause of microbe formation that eventually led to the corrosion. The stagnant conditions provided the environment for bacterial growth, and the residue from the bacteria attacked the metal, causing pits and corrosion.

They reported spending US $96,000 for this problem, in materials alone.

A few of CSI's recommendation based on this experience are:

1. Redesigning the process lines to efficiently keep CIP lines drained and dry to avoid stagnant conditions.
2. Implement higher corrosion-resistant alloys such as C-22® or AL-6XN®. These higher alloys can provide longer life to the process system.

When a comprehensive preventive maintenance program was implemented, the plant’s unexpected downtime was reduced, damage to equipment was minimized, if not eliminated, and the operating life of the systems was extended. Data collected about the equipment being repaired can be utilized for forward planning or for systems upgrades.

Read the article, "Are Microbes Eating Your Sanitary Process System?" to learn more about MIC.