Are Microbes Eating Your Sanitary Process System?

In the food, dairy, and beverage industry, microbiological induced corrosion (MIC), if not properly attended, can become a financial detriment to the plant. If preventive maintenance against MIC is not in place, thousands of dollars worth of equipment and materials can slowly decay. Unexpected downtime can occur, costing a plant up to hundreds of thousands of dollars of wasted raw materials. In a worst-case scenario, it could cause contamination of the food products, affecting the safety of the consumers and damaging the reputation of the company and product.

Microbes, or bacteria, are not corrosive by themselves, but these bacteria can participate in the corrosion process in different ways: changing the environment by replacing a substance with another, partially covering the metallic surface with their biofilms creating local corrosion cells, or inducing the corrosion process by depolarizing the hydrogen at the metal surface. Stagnant conditions can create an opportunity for microbes to grow, and the residue microbes leave behind contribute to the corrosion process.

Current research about MIC shows that aerobic iron bacteria, such as ferrobacillus ferro-oxidans, are the primary microorganisms involved. These bacteria may directly oxidize iron into iron oxides or hydroxides or may lead to the formation of rust deposits with the creation of oxygen concentration cells. With the sulfur oxidizing bacteria, it is the sulfuric acid, produced as a metabolic byproduct, which causes the corrosion.

A wide variety of equipment made from stainless steel is used extensively in the food, dairy, and beverage industry to ensure cleanliness and durability. The surfaces that come in touch with food products are easily sanitized by means of manual or automatic mechanical or chemical cleaning. Clean-in-place (CIP) systems that allow equipment to be sanitized without the need of disassembly are found throughout the industry. Stainless steel, an especially corrosion-resistant material, is used to resist corrosive attack from sanitizing chemicals. However, the highly nutritious foodstuffs processed in the equipment are actually the ideal media for bacterial growth, which can induce a corrosion process on stainless steel surfaces.

In the laboratory setting, three types of evidence are used to diagnose MIC: metallurgical, chemical, and biological. Having three independent types of measurements that are consistent with the mechanism of MIC, one can confirm with utmost certainty the presence of MIC on a sample.

But aside from carefully conducted laboratory tests such as x-ray spectrometry, microphotography, electron scanning microscopy, or biological culture analysis, the presence of MIC can still be properly deduced in the field with acceptable accuracy.

MIC, especially in the dairy, food and beverage industry, is typically caused by just a few conditions. Accumulated foodstuff and water that is left alone in extended periods of time, even in tiny amounts, can become a rich soup of nutrients that bacteria will thrive on. In that environment, when the bacteria attaches to metal surfaces, its metabolic by-products such as oxygen or sulfuric acid, depending on the microbe, oxidizes the metal surface around it, one microscopic piece at a time, until pitting can be obviously seen, or in worse cases, until it breaks through the metal, creating holes. Pitting is typically the first sign of MIC.

This usually occurs in areas that are not properly cleaned by the CIP system. Broken gaskets in equipment allow food and water to accumulate. Watch out for areas where food or water stagnates or pools, such as low lying pipes or dead legs. Accumulated wet foodstuff becomes a rich nutrient haven for bacteria. The residue from these bacteria, in certain conditions, can participate in the corrosion process.

Recommendations

Implementing a preventive maintenance program with a focus on gasket replacement is imperative. Mixproof valves may not leak, but they can harbor bacteria when the gaskets fail and cause expensive valve stem and product failures.

Redesigning process lines to prevent the stagnation of food or other liquids should be considered to prevent bacterial growth causing MIC. Look for dead legs near low points, such as pumps.

Typically, higher corrosion-resistant alloys, such as C-22® or AL-6XN®, show more resistance to MIC than 300 Series alloy materials.

Read this case study to learn how other companies utilized CSI to identify and solve their MIC problems.