9 Corrosion
What should be used to remove corrosion from highly stressed parts?
- A fine stone - fine abrasive paper - DO NOT USE WIRE BRUSHES
To aid the visual inspection of decking corrosion, what can be used?
- Magnifying Glasses - Mirrors - Borescopes - Fiber optics - Other optical inspection tools
Explain Mechanical Corrosion removal relating to aluminum alloys.
- Nylon scrubbers (scotch-brite pads) - Brushing with aluminum Wool or wire brush. - Blasting the surface with glass - Files or powered grinders - sand paper to finish
Specify the effects of the following corrosive agents on aircraft structure. Salts
- Salts are the results of a metallic element combining with a non-metallic element - The result is always a good electrolyte - Magnesium is particularly vulnerable to a corrosive attack from an electrolyte
Specify the effects of the following corrosive agents on aircraft structure Acids and alkalis
- react with metals to form metallic salts - Aluminium is more susceptible to damage from both alkaline than acids
Describe the general identifying characteristics of corrosion on the following metals: Aluminium
- white and generally more voluminous than the original base metal. - general etching, pitting, or roughness of the aluminum surfaces.
Describe Surface oxide film (anodizing) in relation to aluminum alloys
A part is bath in a lead vat containing chromic acid and water. This process forms an oxide film on the part that protects the alloy from further corrosion
What is Nickel or chrome plating in terms of ferrous metals?
A process that produces and airtight coating over the surface that excludes moisture from the base metal.
What is wash primer used for terms of aluminum alloys?
A self-etching primer used on aluminum or magnesium. It is often used to prepare the surface for zinc chromate primer.
In the pulse echo method, what would indicate corrosion?
A shorter spike in the graph, due to the thickness of the material being different from corrosion
What is chromic acid etch, and what is it used for terms of aluminum alloys?
A solution of sodium dichromate, nitric acid, and water used to etch or roughen a surface.
What is epoxy primer used for in aluminum alloys?
A two-part catalyzed paint material used to provide a good bond between a surface and the top coats.
Explain Mechanical corrosion removal in relation to ferrous metals
Abrasive paper and wire brushes can be used the most though is abrasive blasting.
Explain Chemical neutralization in relation to aluminum alloys
After removing all corrosion, treat the surface with a 5% chromic acid solution to neutralize any remain corrosion salts
Specify the effects of the following corrosive agents on aircraft structure. Air (moisture, humidity, acid rain, volcanic fallout)
Air holds moisture, and air above the sea holds high concentrations of salt
What is formed when aluminum is exposed to oxygen?
Aluminum oxide
What is exfoliation corrosion?
An extreme case of intergranular corrosion
What causes Uniform surface corrosion?
Anytime an area of unprotected metal is exposed to an atmosphere that contains contaminants such as exhaust fumes, battery fumes and acidic rain
Why does corrosion easily form in the engine exhaust?
Because of the elevated temperature
How could you tell if copper or brass has corroded?
Blue powder will form on it
What is the limitation of this?
Corrosion can fill the cracks and may be undectable
What is zinc chromate used for in terms of aluminum alloys?
Corrosion-inhibiting primer. Moisture releases chromate ions to inhibit the formation of corrosion
What type of corrosion is Stainless Steels susceptible to?
Crevice corrosion, corrosion cracking, intergranular corrosion (300 series), surface corrosion (400 series)
What process is used to remove carbon deposits that build up on engine crank cases?
Decarburization
an attack resulting from a direct exposure of a bare surface to a caustic liquid or gasous agent. The most common agents causing direct chemical attack on aircraft are : Spilled battery acid or fumes from batteries, residual flux deposits resulting from inadequately cleaned, welded, brazed, or soldered joints, and entrapped caustic cleaning solutions. Which class or corrosion is this?
Direct Chemical Attack
requires a medium, usually water, which is capable of conducting a tiny electrical current . When a metal comes in contact with a corrosive agent and is also connected by a liqiud or gaseous path through which electrons may flow, corrosion begins. The electrochemical attack is responsible for most forms of corrosion on aircraft structure and component parts. Which class or corrosion is this?
Electrochemical Attack
What type of corrosion is Gold susceptible to?
Highly corrosion resistant
What type of corrosion is Titanium susceptible to?
Highly corrosion resistant
What type of corrosion is magnesium susceptible to?
Highly susceptible to pitting
In the ultrasonic method, what would indicate corrosion?
If corrosion is present, it will give off a different frequency to the same metal.
How is galvanizing similar to cadmium plating?
In that if the surface is attacked by corrosion, it will attack the zinc and the zinc will form an air tight oxide layer that protects the base steel. (sacrificial corrosion)
What is the order of polarity of the electrochemical series? (from anodic to cathodic) (will give up its electrons most easily to doesn't give electrons easily)
Magnesium Zinc Clad 7075 Aluminium Alloy Commercially pure aluminum alloy Clad 2024 Aluminium alloy Cadmium 7075-T6 Aluminium alloy 2024-T3 Aluminium Alloy Mild Steel Lead Tin Copper Stainless Steel Silver Nickel Chromium Gold
What should you not use to remove corrosion from magnesium and why?
Metallic tools since magnesium is anodic to almost all metals
Identify what substances metals are converted into during the corrosion process.
Most metal exist as chemical compounds such as oxides or chlorides. They are never generally found in there pure state. And must be refined. Corrosion simply returns metals from there pure state into there natural state as an oxide or chloride.
Describe the general identifying characteristics of corrosion on the following metals: Steel
One of the most familiar types of corrosion is ferrous oxide (rust),
What is the most common form of organic coating used on aluminum?
Paint
What type of corrosion is Chromium (plate) susceptible to?
Pitting (promotes rusting of steel where pits occur in plating)
It is often necessary for dissimilar metals to be held in contact with each. Describe Dissimilar Metal Insulation.
The areas to be joined are sprayed with two layers of zinc chromate primer and a strip of pressure sensitive vinyl tape is placed between the surfaces before they are assembled.
Describe the general identifying characteristics of corrosion on the following metals: Magnesium
The beginning of attack shows as a lifting of the paint films and white spots on the mag- nesium surface. These rapidly develop into snow-like mounds or even "white whiskers."
What is metal spraying?
The process of spraying molten aluminum onto the surface of the metal.
Describe the general identifying characteristics of corrosion on the following metals: Titanium
Titanium is, by nature, highly corrosion resis- tant, but it may show deterioration from the presence of salt deposits and metal impurities, particularly at high temperatures.
How do you prevent galvanic corrosion?
Type of corrosion can be avoided by proper applications of protective finishes, slants and corrosion inhibiting compounds
What type of corrosion is Cadmium susceptible to?
Uniform surface corrosion (used as a sacrificial plating to protect steel)
What is pitting corrosion likely a result of?
Uniform surface corrosion left untreated.
What causes inter granular corrosion?
Usually it has been left to long between the furnace and the quenching process.
When does active-passive cell corrosion occur?
When a salt and water deposit forms on the pass film protective coating and the film is broken corrosion occurs.
Why is pure aluminum considered to be corrosion resistant?
When aluminum is exposed to oxygen, two atoms of aluminium bond with three atoms of oxygen to form one molecule of aluminium oxide. Aluminium oxide forms an unbroken airtight film over the surface of the metal and prevents anymore oxygen reaching the metal underneath and the oxidation rate decreases and almost stops completely.
What type of corrosion is Silver susceptible to?
Will tarnish in the presence of sulpher
What does filiform corrosion look like?
Worms under the paint / puffiness under the paint.
Does the surface need to be smooth for die penetrate inspections?
Yes because it can be hard to remove all the die penetrate off an uneven surface.
What is inter granular corrosion?
an attack along the grain bounds of the material
What is parkerisng
application of a chemical phosphate conversion coating.
After all the corrosion has been removed, what surface treat happens next to magnesium?
chromic acid picking solution
Oxygen Concentration Cell Corrosion forms in _______ (open or closed) areas.
closed
What does pitting corrosion look like?
clumps of powdery salts of corrosion on the surface metal as it is corroded.
What is sheardizing
coat (iron or steel) with zinc by heating it in contact with zinc dust
What causes stress corrosion?
corrosion forms from the cracks due to stress. Can be trans granular or intergrandular
In the cadmium plating of ferrous metals, how is this attached?
electroplated onto the steel
True or false? Electricity can be conducted through the anodized film
false
Where does Concentration Cell Corrosion occur?
in a metal to metal joint or edge of a joint.
Metal Ion Concentration Cell Corrosion forms in _______ (open or closed) areas.
open
Iron oxide forms a porous film which allows more __________ to pass through and oxidize more and more metal until it is completely converted into rust.
oxygen
What are the three typical NDT inspections that can be used to inspect for corrosion?
- Die Penetrate - Ultrasonic 1. Puls Echo 2. Ultrasonic energy - X-ray
What type of corrosion is Tin susceptible to?
Subject to whisker growth
What type of corrosion is Copper Base, brass, bronze susceptible to?
Surface and intergranular corrosion
What type of corrosion is Low Alloy Steel susceptible to?
Surface oxidation, surface pitting, intergranular
What type of corrosion is Aluminum susceptible to?
Surface pitting, intergranular, exfoliation, stress corrosion, fatigue cracking and fretting
What type of surfaces does filiform corrosion occur on?
Surfaces that have an organic coating.
What are the two classes of corrosion?
1. Direct Chemical Attack 2. Electrochemical Attack
What are the two types of Concentration cell corrosion?
1. Oxygen Concentration Cell Corrosion 2. Metal Concentration Cell Corrosion
What are the three basic steps to removing corrosion?
1. Remove as much of the corrosion as possible 2. Neutralize any residual material 3. Restore the protective Surface Film
What two things are important when removing corrosion form highly stressed parts? in terms of ferrous metals
1. Should be removed immediately 2. The minimum amount of material possible should be removed
What three things are required for a electrochemical attack to occur?
1. There must be an electric potential difference (voltage difference) within the metal or between metals. 2. There must be a conductive path between the areas of potential difference. 3. There must be some form of electrolyte (a conductive liquid or gas) covering the areas of potential difference.
What is sacrificial corrosion?
A method of corrosion protection in which a surface plated with metal a metal is less noble than itself. Any corrosion will attack the plating rather than the base metal
What is fretting corrosion
Fretting corrosion forms between two surfaces that are held tightly together yet still move slightly against one another. This rubbing action removes the oxide layer on aluminium parts, the exposed aluminium quickly forms a new oxide layer which is also worn away.
When does galvanic corrosion occur?
Galvanic corrosion occurs any time two dissimilar metals make contact in the presence of an electrolyte.
Steel parts such as firewalls are typically treated with a coating of zinc, what is this process called?
Galvanising
What forms when a Ferrous metal is exposed to oxygen?
Iron oxide forms, commonly known as rust
Where would you use a die penetrate to detect corrosion?
Stress corrosion cracks
Specify the effects of the following corrosive agents on aircraft structure. Salt spray
Salt spray from sea water cause a much more rapid corrosion and than fresh water.
What is typically used for abrasive blasting?
Sand, aluminum oxide, or glass beads
Specify the effects of the following corrosive agents on aircraft structure. Organic growths
Scum (microbial growth) forms in fuel tanks from water that is entrained in the fuel. This layers the fuel tank and provides a place for corrosion to form.
What is done to prevent stress corrosion?
Shot-peening
What is a indication of fretting corrosion?
Smoking rivets
What does Uniform surface corrosion do to the metal?
react with the metal and convert microscopic amounts into corrosion salt over the entire surface giving it a uniform dull appearance. If these deposits of salt aren't removed, pits of corrosion will form due to the localised anodic area.
What is most common cause of filiform corrosion?
the wash primer has not been properly cured.
What is hard chrome used for in terms of ferrous metals?
used to form a wear resistant surface on piston rods, cylinder walls etc.
When does oxidation occur?
when ever a metal is exposed to oxygen
Where does Oxygen Concentration Cell Corrosion occur?
where water is trapped between the lap joints.
How could you tell if lead has corrode?
white powder on it