Day 36: Introduction to Corrosion
- Importance of Corrosion
- What is Corrosion? Some theory.
- The four things that are required for corrosion
- Types of Corrosion
Environmental Degradation of Materials
- Materials are “attacked” by their operating environment.
- We will focus on the degradation of metals. This is called corrosion.
- Some attention will be paid to polymers, but none to ceramics.
- In metals, corrosion is produced by the loss of actual material, which leaves the piece as an ion in solution, and is carried away by an electrolyte.
- Rust is a symptom of this problem in steel, but there can be corrosion without it.
Importance of Corrosion
- The impact of corrosion on society is very significant.
Direct Costs of Corrosion:
Nearly 300 G$ in 1998. Clearly, they rise proportionally till today.
Between 3-5% of the Gross Domestic Product.
When Protective Coatings Break Down, things can get ugly.
From Corrosion Doctors web site
This obvious, up front relatively uniform corrosion is relatively benign. We see it for a long time before it hurts us. Not all corrosion is so nice.
Atlantic Southeast 529, 8-21-1995
Lead wool added for balancing
A Corrosion Disaster
The Safety Board concludes that one of the four blades from the left
engine propeller separated in flight because a fatigue crack that originated from multiple corrosion pits in the taper bore surface of the blade spar propagated toward the outside of the blade, around both sides of the taper bore, then reached critical size. (See Section 1.16.1.)
Results of investigations conducted in two previous propeller blade
failures in 1994, one in Brazil with this model blade and the other in Canada with a similar model blade, indicated that corrosion was produced when entrapped moisture reacted with residual chlorine in a bleached cork used to retain the lead wool in the taper bore hole of the propeller.
Point: Corrosion is subtle and very hard to detect
So, What exactly is corrosion?
- Corrosion is an irreversible interfacial reaction of a material (metal, ceramic, polymer) with its environment which results in consumption of the material or in dissolution into the material of a component of the environment.
- Chemistry is at work. We are talking about a certain class of chemical reactions between a metal and the environment.
Example – The Daniell Cell
- This example illustrates some of the basics of corrosion.
- On the surface of the Zn bar we have the following
- On the surface of the Cu bar we have the following
Note the current path. The salt bridge provides for ion exchange.
Dissimilar Metals have Galvanic Potential
Any voltage, even if small will produce corrosion damage over time.
Clearly dissimilar metals will create a corrosion cell. The anodic metal will be damaged.
Please note the presence of stainless steel
- Yes, under certain circumstances, stainless becomes active.
- Factors: (These are bad for any metal!)
- Low aeration in water
- Low velocity water
- Presence of Cl-. Chlorine is one of the worst offenders in promoting corrosion.
- Here is a typical reduction reaction involving hydrogen ions in solution. Note that the H gains electrons.
- Here is an oxidation reaction. Fe is the symbol for iron. Note that metal looses electrons.
These Reactions want to occur in Pairs
We are assuming that the Fe is surrounded by a weak acid in which H+ ions are abundant.
This acid is called an electrolyte. It provides a home for the dissolve Fe+2 ion.
Note that there has to be an internal movement of electrons through the Fe.
Where is the Cathode?
- At the location at which hydrogen is being liberated, we have a local cathode, associated with what is called a hydrogen overvoltage.
- Summary: What’s needed for Corrosion
- An anode. This is where the damage occurs. Oxidation takes place.
- A cathode. Here’s where the reduction reaction takes place.
- An electrolyte. (Almost any moisture will do.)
- A current path between the cathode and anode.
- Anode: (Metal basically dissolves in the electrolyte.)
- Cathode: (This is a very common reaction!)
Surfaces near high O2 concentration are cathodic!
Types of Corrosion
- Uniform - common surface effect.
- Galvanic - dissimilar metals.
- Crevice corrosion.
- Errosion corrosion.
- selective leaching. De-zincification of brass
- Stress corrosion.
- Hydrogen embrittlement
- This one is common in steel that is unprotected by any surface coating. Most noticeable. Surface effect, leaving rust on the surface.
- The good thing about this, if there is one, is that the corrosion is widely spread around.
- The more dangerous forms of corrosion are:
- Highly localized, concentrated.
Steel screw in Mg
Steel screws and brass
Dissimilar metals, the damage occurs at the anode.
This is a concentration cell in action. Notice how the damage occurs in out of sight places.
- This is similar to crevice corrosion. It is based on low oxygen concentration at the bottom of the pit.
- This is very common in materials that protect themselves with a passive layer, i.e. stainless. Also, aluminum.
Highly localized. Goes deep into the metal.
Chloride ions find their way into the pits, exacerbating the situation.
- Sometimes called stress corrosion cracking.
- Ingredients: (1) tensile stress in the metal (2) corrosive (electrolyte) environment.
- Accelerators: presence of Chloride ion and high temp.
- Victims: Stainless steel is unsafe in water above 50C and over a few ppm of chloride, if any tension exists. Others: mild steel in alkaline environment, copper alloys in ammonia env.
- The anode is the stresses region.
SCC in Stainless Steel
Failure is along grain boundaries.
- This is a segue from the previous. It is closely related.
- Again, stainless steel is the ideal victim here. The problem is triggered by improper heating, and often this comes with welding. Carbides of chromium form in the grain boundary regions.
- The chromium is tied up in the carbides. It can’t protect by forming the passive layer.
- PLUS, there is a dissimilarity in metals producing a small but definite galvanic corrosion.
- Exfoliation corrosion in Aluminum that has been heavily worked, such as in extrusion.
- Corrosion products start to build up in between the long elongated grains, separating them and leadin to increased corrosion propagation through the metal.
- Another example of microstructural corrosion.
- In an alloy system, one phase may be anodic with respect to another phase.
- Example: dezincification of brass.
- Example: graphitization of cast iron.
- This is caused by the impingement of a high velocity turbulent flow on a surface.
- The flow is often multi-phase. This means there can be entrained solid particles, or even gas bubbles, as in cavitation of a propeller.
- The flow will carry away any protective layer that was intended to protect the material, and even abrade the flow surface.
- This is not exactly galvanic corrosion, but it definitely is a form of environmental attack.
- Hydrogen atoms diffuse into the metal from outside. Deep in the metal, they combine to form H2 gas or combine with C, if present to form CH4.
- The pressure in this internal pockets of gas is enough to initiate cracking.
- The metal is already seeing a lot of tensile stress.
- Normally ductile high strength metals, particularly steels, are not so ductile anymore because of these internal cracks.
Where does the Hydrogen come from?
- Arc welding can a source. Hydrogen might be released from the electrode.
- Galvanic corrosion can produce hydrogen in a reduction reaction.
- Sour gas wells
- Hydrogen storage (You just don’t use high strength steel!)
- Protection of the Anode. (Passivation)
- Reduce the activity of the cathode and or electrolyte. (Polarization)
- Sacrificial Anodes
- Impressed Voltages
Passivation of the anode
- We have two examples already. Stainless and aluminum.
- A thin oxide layer forms on the surface and isolates the metal from the environment.
- Zn, Mg, Cu and Ti are also capable of passivation under normal conditions of operation.
- Steel will also passivate in the presence of an alkaline environment, such as rebar in concrete.
- Corrosion inhibitors. Some of these, such as the chromates, are capable of coating a steel and passivating it.
- Coatings, paints, etc.
- This is an effect which reduces the actual chemical potential driving the cell. If the thermodynamic force driving the ion into solution is reduced, this is polarization.
- Easy example. By lowering the electrolyte temperature, we find that it is usually less corrosive. Diffusion of ions is slowed.
- Inhibitors are chemicals which slow corrosion. Some of them do this by promoting the polarization of the cathode.
- Galvanization of Steel
- Dip steel sheet in molten zinc. Get a pretty thin coating.
- Zinc will be anode. Steel exposed by crack is the cathode. Since we have a huge anode having to be served by a small cathode, corrosion rate will be slow.
Tiny cathode (steel)
Large area anode (zinc)
An example of a favorable area ratio. Bad deal: huge cathode, tiny anode
- Zinc is attached to the steel hull of the vessel.
Sacrificial Anode for a Pipeline
By imposing a voltage which causes electrons to flow towards the object to be protected, we make it less anodic and protect it from corrosion damage.
- Swelling and Dissolution (Solvents)
- Bond Rupture
- Radiation (UV and higher)
- Chemical Reaction Effects (Oxygen and Ozone)
- Thermal Effects
- Exposure to UV can result in deterioration of appearance and mechanical properties.
- UV photons have sufficient energy to break carbon-carbon bonds
- UV + Oxygen is photooxidation
- The property degradation is due to
- Chain scission (reduction in molecular weight)
- Crosslinking (loss of ductility)
- Induced Crystallization
Free Radical oxidation of UHMWPE tibial implant. Could happen in vivo or in vitro. Vitamin E has been tried to deal with the free radicals.