Chloride attack on stainless steel.

A guide to leaks, pin holes and heart ache in the food industry.

 Presented By N.E.M Business Solutions

Any compound capable of donating free chlorine ions (Cl -) to an aqueous (water-based) solution has the potential for causing failure in stainless steels. The chlorine ion is extremely electronegative, and therefore very reactive with certain compounds and elements. This reactivity is part of its usefulness in certain situations, but becomes a double-edged sword where stainless steel is concerned. Chlorine can be introduced into a piping system in many ways, but the most common seen in food industry applications are as salt (sodium chloride) and in chlorine-based sterilising solutions such as bleach (sodium hypochlorite.) Salt water (brine) is known to corrode stainless steel, as is bleach. Evidence of severe corrosion in seawater applications is frequently found in textbooks dealing with corrosion. The following picture shows severe corrosion in a 304L stainless steel tube that carried water used for cooking cheese. The salt in the cheese leached into the water, creating an ideal environment for corrosion.

Chloride Attack on Stainless Steels

Chloride-induced corrosion is not bulk corrosion. We are all familiar with one of the most common forms of bulk corrosion: rust. When iron rusts, the attack is fairly uniform over the surface exposed to the corrosive environment. Chloride attack of stainless steel is exactly the opposite crevices and pits form and grow perpendicularly to the surface being attacked, rather than spreading out evenly as rust does. Some areas may appear essentially untouched by the corrosion, while others will be severely attacked. This means that thicker tubes and pipes will not necessarily last much longer than thin ones before failing due to chloride induced corrosion.

This is an example of pitting:

Stainless steels have excellent corrosion resistance. Water supplies will usually have chlorine or hydrochloric acid added to prevent bacterial growth, it is important to use a material that will resist corrosion by such harsh chemicals. 316 stainless is highly effective in resisting this corrosion.

Stainless steel dissolves very slowly in water, even “RO” (reverse osmosis) water, so only a very small amount of chemical compound elements are added to the water. Using plastic, copper, or iron allows all kinds of unknowns to be added to the water. Plasticiser, from certain plastics, can be leached into the water system, especially when aggressive RO water is used.

Stainless steel is a very clean material and can be sanitised easily. If dead spaces are minimized and surfaces polished to eliminate crevices and pits, bacteria growth is minimized.  This is especially true when the piping system is flushed or has continuous water circulation at velocities high enough to cause turbulent flow. Turbulent flow is important because the turbulence creates a scouring of the pipe surface. Low flow or laminar flow leaves a stagnant film of water next to the pipe surface and a biofilm can form.

 
Since corrosion resistance is a primary reason for the use of stainless steels, a basic understanding of the types of corrosion and how they occur is important.

Definition:
Corrosion is the degradation of a metal by its environment--it literally means to "gnaw away".  Most metals do not exist as a solid metal piece of material. In their natural state, they exist in the form of oxides. These metal oxides (or other metal compounds) must be refined to create the pure metals or alloys which become useful structural materials that can be used to build things. 

Pure metals and alloys have a much higher energy state and there is a natural tendency to return to their lower energy state. Corrosion is the process nature uses to return metals to their original state. The rate of corrosion depends upon the environment and the type of material. It can be very rapid in a highly corrosive environment or take thousands of years in a slightly corrosive environment.

Corrosion, whether in the atmosphere, underwater, or underground, is caused by the flow of electricity from one metal to another metal, or from one part of the surface of a piece of metal to another part of the same metal where conditions permit the flow of electricity. For this to occur there must be a moist conductor or electrolyte present for the flow of energy to take place.

Different types:
There are many types of corrosion that can affect metals.  They include:
general, electrochemical, galvanic, pitting, crevice, impingement, erosion, stress, biological, and intergranular. 

The most likely are:

• galvanic / electrochemical
• pitting and crevice
• intergranular 
• biological
   

Galvanic/Electrochemical:
The diagram below illustrates a cell showing the corrosion process in its simplest form. This cell includes the following essential components.

• A metal anode
• A metal cathode
• A metallic conductor between the anode and the cathode
• An electrolyte in contact with the anode and cathode, but not necessarily of the same composition at the two locations. 

The components are arranged to form a closed electrical path or circuit.

Suppose that the anode is iron, the cathode is copper, and the electrolyte is water containing mineral salts. The anode is negatively charged and the cathode is positively charged. This difference in charge (voltage) provides potential voltage, which is the driving force for current to flow in the cell.

Since the iron in the test cell is negatively charged, and the copper is positively charged, there is a potential voltage difference which causes a flow of electricity. The anode will give off iron ions in the form of rust (corrosion), while hydrogen gas would be produced at the cathode and no destruction will occur. Corrosion occurs only on the anode. The rate of corrosion in the cell will be dependent upon the relative sizes of the anode and cathode and the potential difference between the anode and the cathode.

If, for instance, the anode was very small and the cathode was large, the rate of corrosion would be very rapid. The opposite would be true if there was a very large anode compared to the cathode. If the anode was nickel and the cathode brass, there will be very little corrosion, because the voltage potential difference will be slight.

Galvanistic corrosion occurs when dissimilar metals are used. When the materials have a large difference in voltage charge they are more likely to corrode. For instance, if aluminium was the anode and silver was the cathode, the aluminium would corrode very rapidly, because they are dissimilar metals. Of course, to corrode there must be an electrolyte present—water, moist air, etc. Naturally the closer together the metals or alloys are in the galvanistic series, the less likely that corrosion will occur.

Galvanic Series of Metals:

The metals below are arranged according to their tendency to corrode galvanically.   Metals with negative voltage charges (anodic–least noble) are listed first, followed by metals with positive charges (cathodic–more noble).

The

Pitting and Crevice:
These localised attacks on stainless steel can produce surface pitting and crevice corrosion. Most pits form when there is an inclusion or there has been a breakdown of the passive film. Crevice corrosion occurs at locations where crevices exist, such as threads, machining grooves, tears, metal lap joints, etc.

The illustration below shows how corrosion occurs at a crevice created by a lap joint. At the edge of the lap joint, movement of water (electrolyte) flushes away metal ions resulting in a lower metal ion concentration. The space between the two pieces of metal is stagnant and there is a higher concentration of metal ions, allowing corrosion to occur at the edge of the mechanical joint.

An oxygen concentration cell may also form if there is a depletion of oxygen in the dead space in the lap joint. If the material is stainless steel and there are high levels of chlorine in the water, the chlorine will attack metal in the dead space between the two pieces of metal, breaking down the passive film.

Since there isn't any oxygen available to regenerate the passive film, the stainless becomes active (anodic) in this cell and the rest of the stainless stays passive (cathodic) because the passive film remains intact. With this lap joint in water (electrolyte) conditions are right for current to flow and corrosion occurs in the crevices formed in the lap joint. 

Concentration cells can form in any crevice in watering systems and corrosion is more likely to occur with the use of chlorine or hydrochloric acid. Corrosion may be accelerated if there are large amounts of organic material and very low levels of oxygen in the water along with the use of chlorine. Oxygen is necessary to maintain the passive film.

Edstrom Industries uses 316 stainless steel to prevent pitting problems due to the use of chlorine. The molybdenum in 316 helps to stabilize the passive film, although excessive levels of chlorine will corrode even 316 stainless steel.

Intergranular:
This type of corrosion may occur next to a weld if the carbon content of the stainless steel is too high. When stainless steel is welded, material next to the weld reaches a temperature of only 800° to 1500°F. At these temperatures, the chromium and carbon form chromium carbides. Chromium carbides deplete the chromium at the weld interface and sensitise the material, making it subject to corrosion.

If a weld interface is deficient, it cannot maintain the passive film. This area becomes anodic, while the rest of the material is cathodic. When the material is in water or moist air (the electrolyte), current will flow, resulting in corrosion (rusting) at the weld interface. By reducing the carbon content, we can prevent carbides from forming. For this reason, Edstrom Industries uses 316L or 304L stainless steel when welding to avoid intergranular corrosion.

Biological:
When a metallic surface is immersed in water, a biofilm will begin to form if there is any bacteria in the water. A biofilm is a microbial mass composed of aquatic bacteria, algae, or other microorganisms. The biofilm begins when organic material is absorbed onto the surface of the metal. The flow of water transports microbes to the surface, and the microorganisms attach and then grow, using nutrients from the water.

When the microorganisms grow, oxygen is excluded, which creates a place where the passive film may break down. With the breakdown of the passive film, the site becomes anodic with the likelihood of corrosion. Biofilm formation is most likely in spots where the flow of water is low, such as voids, crevices, and thread joints.

Steps in biofilm formation.  Formation is initiated when small organic molecules become attached to an inert surface

(1) Microbiological cells are absorbed onto the resulting layer

(2).  The cells send out hair like exopolymers to feed on organic matter and attach themselves to the surface

(3), adding to the coating

(4).  Flowing water detaches dome of the formation

(5), producing an equilibrium layer

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