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The Hot Topic of Combi Oven Corrosion - Part 1

In the last year, corrosion of Combi steam ovens has become one of our most common calls regarding water quality problems. 

Corrosion is also the most complex of all water-related issues simply because there are so many Crevice Corrosion from Spot Weld Jointcontributing factors.  Due to the complexity of this topic, we will offer a series of postings to take a more detailed look at several water-related issues.  The following is an overview.

The causes of corrosion in a steam oven are varied and the degree of the problem is amplified by combining factors.  The reality is that without water, you would not have corrosion.  However, the water by itself might not be the cause. 

Potential causes of corrosion in a Combi oven can be and are not limited to the following:

  • Chlorine or Chloramines in the tap water
  • Elevated Chlorides or Sulfates (dissolved minerals)
  • Elevated levels of Sodium (in the water or from the food cooked)
  • Types of food or patterns of cooking
  • Galvanic and/or Electrolytic (Dissimilar metals or improper grounds)
  • Food Deposits
  • Improper Cleaning tools and/or chemicals
  • Metal fatigue
  • Thermo-galvanic (Temperature differentials within metals).

It is important to know that stainless steel will corrode due to all of the causes above and others. However, these problems can be avoided or significantly minimalized through proper equipment care and operation, and managing water quality.

An unusual example that we have seen in various locations and with different brands of ovens is where two ovens are sitting side-by-side of the same make and model. One has been in service for less than three years and the other for twice as long.  The older oven shows no signs of corrosion and the three year old oven looks like it is ready for the scrap yard with significant corrosion.  The ovens had the same water supply, same operators, and were exposed to the same cleaners.  Some in-depth investigation revealed that the three year old oven was operated at low temp (straight) steam 100% of the time; and the older oven in combo and in convection with no steam alternatively throughout the day, allowing the oven to dry out.

It is not uncommon for operators to see rust starting in corners or sliding racks and mistakenly use an acidic cleaner to try and remove it.  This is comparable to throwing gasoline on a fire to try and put it out.  Acid cleaners and de-scalers will accelerate the corrosion process increasing the amount of rusting.  We also have seen operators use wire brushes to clean off deposits of food, scale or rust.  Again, this only adds to the problem by scratching the surface of the stainless steel and leaving behind microscopic particles of iron in the scratches.  These iron deposits act as a catalyst for new corrosion.

Major contributors to corrosion in ovens are chlorides and sulfates in tap water. Chlorides are naturally occurring in tap water at different levels throughout the country. Chlorides levels can be increased due to the concentration of surface waters and the influx of chloride containing effluents in the waste stream or runoff.  Chlorides will concentrate in microscopic cracks, crevices and joints of your oven and will cause “Crevice Corrosion.”  Most of the corrosion in Combi ovens is typically Crevice Corrosion due to the depletion of oxygen in the crevice, a shift to acid conditions in the crevice and a concentration of aggressive ion species (chlorides) in the crevice.  It is important to know that a carbon or sediment filter or a Weak Acid Cation resin based filter will NOT reduce chlorides or sulfates in the tap water.Crevice Corrosion from Metal Fatigue

How can a Foodservice operator maximize the return on his cooking investment?  We recommend a multi-faceted approach that includes the following best practices:

  • Consult with a reputable manufacturer to select the right equipment for your needs.
  • Make sure your water quality meets the manufacturer’s recommendations year round.
  • If Hardness, TDS, Chlorides, Sulfates, Chlorine or Chloramines exceed allowable levels obtain water treatment equipment capable of reducing the contaminants to acceptable levels.
  • Use only approved cleaners and tools to clean the oven. Never use steel wool or steel/brass bristle brushes or scrapers.
  • Make sure the oven is installed according to manufacturer's specifications.
  • Learn the best cooking methods and utilize the different cooking options to provide the highest quality food and optimize oven life.
  • Alternate cooking methods to allow the oven to dry out and avoid being wet all of the time.
  • Avoid dissimilar metals in plumbing connections.  For example, never use galvanized steel pipe or directly connect steel pipe with copper or brass.
  • Avoid allowing food deposits to trap moisture between the food and the surface of the oven.

This list is not intended to be all inclusive, but just some informative ideas to provide operators with a little more insight into the growing problems with corrosion.   Do not think of these steps as individual solutions but as a synergistic strategy to help minimize the potential for corrosion problems with your Combi oven. 

Agree, Disagree or Have More Questions?  Leave a comment, we'd love to hear from you.

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