Why: To help us better maximize capital spending, plant safety and product quality, by disseminating Corrosion Science and Engineering Technology.
Corrosion is a complex subject and this course will help you to understand the issues in materials selection. Specialization in this subject is required to fully take into consideration all of the possible options for combating or avoiding corrosion. Failure to recognize corrosion beforehand can lead to serious business issues later on.
By giving you background in what causes corrosion and how it manifests itself in the plant we can start to avoid significant business issues in the future.
Recent corrosion issues I have been involved with. What causes corrosion from a chemistry perspective is next. Followed by a discussion on “Passivation” which is the basis for corrosion resistance of stainless steel. A few slides on electrochemistry is next and it will allow you to understand the critical parameters that corrosion engineers study to select a engineering material for a given chemistry. A exercise will be given at the end of the first section on corrosion chemistry.
The second section goes into what the forms of corrosion look like. Examples of each form are displayed in picture format with the causes and remedies outlined. There are two exercises in this section, one on localized corrosion and another on high temperature corrosion.
The final section begins with a process for selecting engineering materials. It is basically a thought process that engineers could use in selecting materials of construction for a plant. Corrosion testing is an important part of materials selection, often times we neglect this subject; a corrosion testing example is given next. The equipment design section includes principles to avoid corrosion both from a process as well as equipment standpoint. The inspection section will help you and the plants to avoid significant corrosion issues. A brief discussion follows on what formulators can do to help us avoid corrosion. The last section summarizes key points on construction materials and what some of the major “pitfalls are”.
Most of the engineering materials compatibility issues are with stainless steel. The examples shown in the next few slides are all “stainless” issues. Examples include processes from oral care, to softsoap, to base plant liquids making. This will put to rest the notion that stainless steels do not corrode.
Macro view of a valve that was installed in a high speed filling line. The equipment supplier changed the metallurgy for this part from austenitic 316 SS. Martensitic stainless steels are not as corrosion resistant as austenitic stainless steel.
Important to notice how a small pit on the surface when shown close up , magnified, is very significant. The passive film on the surface at this point is dissolved and metal is basically going into the product. The concept of passivity will be explained in more detail. It is what gives stainless steel its corrosion resistance.
A spring is worked and drawn putting a lot of mechanical energy into it. When this is done, the potential for corrosion increases dramatically. Coupled with the fact that spring is made out of a lower alloy stainless steel it becomes even more susceptible to corrosion. In this case the process viscosity needed to be adjusted using sodium chloride and when the product was introduced to the package with this check valve spring corrosion occurred. More about this mechanism of corrosion will be explained in Section 2.
As you can see from this picture corrosion can have a significant impact on manufacturing. In this case a soap making column actually deformed. Leaks from the process side to the water side caused significant contamination issues. This column needed to be replaced with an emergency work order.
In another key chemical processing facility a mixer drive shaft assembly and blades failed due to corrosion. The corrosion on the shaft and the blades had gone undetected and therefore the plant was caught by surprise. Half of the production was lost for about a week until a new assembly could be fabricated and then put back in service.
This is a picture of what an underarm gel can do when in contact with a seal. This issue was addressed before the product was launched.
In making liquid detergents, we make the base by sulfonation. We start with the ignition of sulfur at high temperature forming sulfur dioxide, that is then converted to sulfur trioxide and then reacted with an alkane to form the base. At the stage of burning the sulfur the temperature can go as high as 1300F. You can see from the photograph that the 321 stainless steel did not hold up. And corrosion progressed to the point where it perforated the piping in a couple of years of service.
This is a table of the stainless steels that we have had corrosion issues with recently. As you can see even 316L is not immune to corrosion in soap making. Each of these events were significant and we should try to avoid these types of issues in the future.
The chemistry of why a solid reacts with a liquid can be quite complex. However, alot of the reactions are redundant everywhere. It is these reactions and the concept of a corrosion cell that can be much more easily explained and introduced. This is the objective of this section.
It is important to understand that what we are talking about is the reaction of solids with liquids or gases. And although corrosion directly refers to metals, it is expanded (these days to refer to ), or include other materials. Such as plastics, elastomers, FRP, and concrete. There is a society called NACE, the international society of corrosion engineers, who study and examine the effect of all kinds of environments on many different materials.
The beginning of definitions in the corrosion of metals. All four of these elements need to be present for the corrosion reaction to occur.
Oxidation- Is the damage done to the material, how corrodes. It is what we want to avoid in Oral care equipment for example as it contributes contamination. Reduction- the other half of the reaction cell. It does not enter into the metal contamination equation.
Showing the half cell oxidation reaction, iron going into solution. Oxidation reactions can also be from other elements such as chrome, molybdenum and nickel. Some metallic elements are known to cause health issues
Most common reduction reaction, the reduction of Hydrogen ions to form H2 a gas molecule
A corrosion cell showing the four elements together, anode, cathode, electron flow path ( arrow) , and the electrolyte reactions. Fe + OH -- Fe (OH)2 , H+H--H2. Positive charged ions react with negative charge ions.
Introduction to the concept of corrosion reactions driven by different types of metals. In this example copper and iron , with the iron becoming the anode and copper the cathode. The reason for this is that different elements have different affinities for their electrons ( Atomic scale ).
If we block or eliminate any of these four elements we can stop the oxidation or corrosion process. The most common way is to eliminate the oxidation reaction through a higher alloy or coating
Common corrosion reactions on carbon steel. As you can see adjusting the pH to a higher one, Alkaline , we can slow down the reaction process. In this case Hydrogen needing to 2 electrons to form vs. the 4OH- that needs 4 electrons to form. In other words the kinetics are faster with hydrogen ions vs. Oxygen.
pH is an important driver in oxidation and reduction and therefore a piece of data that is important in materials selection.
This is showing the in acids the main anodic reaction is hydrogen ion reduction.
Depending on the chemical and the temperature at high pH, above 10, the most common engineering issue is Stress corrosion cracking. For example in sodium hydroxide issues with stress cracking occurs with carbon steel at relatively low temperatures. Consistent temperatures above 110-120 F with concentrated NaOH and welded carbon steel can cause caustic embrittlement. The main issue with high pH is cracking of steel. For Colgate issues with NaOH at high temperatures are cracking in welds etc.
This shows that the elements corrode at different potentials. The more positive the value the more energy it takes to corrode. For example copper will not generally be attacked by low pH , but when aerated at low pH it will corrode.
Another name for potential-pH diagrams are Pourbaix diagrams. Basically they are corrosion phase diagrams. They can be used to determine the the reactants and products in a given set of circumstances. Basis for understanding the effect potential energy and pH.
Fluorine is the most electronegative and reactive of the elements. Elemental fluorine becomes very corrosive when wet because of formation of hydrofluoric acid by hydrolysis. Chlorine in water forms a mixture of hydrochloric and hypochlorous acids. The latter being very oxidizing which makes it corrosive. Like fluorine and chlorine the corrosivity of bromine is greatly affected by its water content. In water is hydrolyzes to hydrobromic acid. Making the long story short halogens can play havoc with stainless steel.
In general as the temperature rises 10 C or 15 F degrees the corrosion rate doubles. In an enclosed system the corrosion rate eventually peaks and cannot go higher because of oxygen consumption. An rule of thumb temperature to keep in mind is 140 F. Above that temperature you have to be more concerned about corrosion issues and below not as much. Use it as a guide line temperature. For example at 140 F is the temperature that stress corrosion cracking becomes an issue with stainless steels in chloride service.
An example of the first case would be fuming sulfuric acid ( Oleum). I have seen tanks where the design thickness of Oleum tanks are ¾” thick. Although oleum is extremely hazardous to humans it is not corrosive even though it is a very strong (100+percent) sulfuric acid. The reason being it forms a ferrous sulfate film rather easily. The film prevent the further corrosion of the base metal. The corrosion resistance carbon steel is acceptable up to 70 % NaOH at 80 C, 175F. Except for SCC. After that the corrosion rate increases dramatically. Nitric acid although a very strong mineral acid it can be handled by stainless in almost all concentrations because of the very strong passive film it forms with it.
Trace compounds can cause havoc in plants operating at elevated temperatures, in particular above 140 F. ClO3 has caused problems in heat exchangers in glycerin recovery for example. Mainly pitting corrosion.
A separate section on passivity is needed because about 90% of our manufacturing facilities are made out of stainless steel such as 316L. It is the passive nature of stainless that allows us to manufacture our sensitive products , such as toothpaste.
It should be said however, that stainless steel with a minimum of 12% chromium is needed and that the passive nature of this film forms naturally in the air.
This a basic diagram showing the steps in making stainless steel. It starts out with melting the basic metallic ingredients iron , chrome and nickel at high temperature. Then refining the mix by removing impurities such as sulfur. The molten mix is then poured into a mold where an ingot of hot steel solidifies . Then a process of working the steel into various shapes begins with the forging process to make bar and plate . These large billets are then further worked at lower temperatures, cold working , to form pieces that are used by machine shops to make parts.
Depending on the type of stainless steel that is made, i.e. austenitic, martensitic, etc the parts are heat treated at various temperatures . During this step and other working steps a scale forms on the stainless steel. This scale is removed during the pickling step . Pickling basically dissolves all of the oxides to remove them. This will be explained further on a electrochemical diagram. Sulfuric acid is generally used for this step. Following this step a passivating step is used to form the chromium oxide film on the stainless steel with nitric acid. It is the chromium oxide film that gives the steel the film that makes it stainless.
For example there are basically three different families of stainless steel. They austenitic, martensitic, and ferritic. The martensitic grades are quenched to form martensite. Martensite is a crystalline structure that is quite hard and is used for parts that need wear resistance. However, the martensite phase is not as corrosion resistant as austenite. The austenitic alloys such as 304 and 316 SS are more corrosion resistant. The ferritic alloys are generally not used for structural chemical process applications. Other alloys exist such as the higher grades of austenitic and duplex alloys. The point is that not all alloys are equal and they vary in resistance to oxidation or chemical attack.
Surface condition is important for cleaning and sanitization, but it is also important for chemical resistance. Electro polishing for example will provide a smooth surface for cleaning but it also can sacrifice chemical resistance. Looking at this chart you can see that the most chemical resistant surface is one that is mechanically polished passivated and not electro polished.
This oxide film is what makes stainless corrosion resistant. The corrosion resistance will vary though based on many factors. The key factors being the concentration of alloying elements in the stainless steel and the chemical environment.
Citric acid will not cause localized corrosion of stainless steel so it is safe to use on all types of stainless steel. This acid acts to grab iron off the surface. By removing free iron we reduce the chance for localized corrosion at those points. Localized corrosion could lead to micro contamination at those points.
This treatment is a cocktail that removes scales and passivates the metal at the same time. It should be used for tints or scales that are tough to remove like from welding. It is a powerful cleaner and should be used only under controlled conditions.
Electrochemical measurements are a way of finding out how reactive a metal surface is in a given chemical environment. It basically tells you if corrosion will or will not be an issue. It does need analyst interpretation which is the downside. The upside is that the technique is quick and you do not have to wait weeks for an answer to a corrosion question on chemical compatibility.
This is a general representation of how stainless steel behaves electrochemically. It is a plot of corrosion potential and current. Looking at it appears to look like a whale. The bigger the head of the whale the better the alloy will be in that service chemical. When we pickle a stainless steel “remove the oxide scale” we are in the mouth of the whale potentially speaking. When the potential is in the upper head of the whale we are at the pitting potential. Pitting meaning localized corrosion can occur. A position where we do not want to be.
This shows the whale vs. the shark. 316 being the whale and 440B being the shark. Notice the large mouth on the curve of the shark and the small head. What this means technically is that the 440 B alloy will not be as corrosion resistant in Nitric acid. This is the reason we have to be careful with nitric acid, some alloys and welds may not compatible with it.
More importantly than the nitric acid compatibility issue is the effect oxidizing potentials can have on an alloy. In this case the pitting potential for 440 B could be exceeded causing localized corrosion much more easily than with 316 SS.
Anode, cathode, electrolyte, metallic pathway. Oxidation and reduction. Oxidation .. Loss of electrons, Reduction gain of electrons. Temperature, pH, velocity, chemistry. Trace elements, electrochemical potential Pickling you are using acid concentrations that actively remove all of the passive film and any other oxides. Passivation is only removing metal contaminants or repairing the chromium oxide by using nitric acid. Citric acid chelates free iron of the surface and therefore makes the oxide film on the stainless more resistant to chemical attack.
Picture of a valve that corroded after one week of service. The valve is made out of a martensitic stainless steel. The exact specification is 440B. This alloy was selected based on its wear properties.
Close-up of Rotary Valve Showing the severe localized attack