P04 Design for reducing the corrosion rate Flashcards
Define Corrosion
Corrosion is the gradual destruction of material (usually metals) by chemical reaction with its environment. (electrochemical oxidation of metals in reaction with an oxidant such as oxygen e.g. rust). Corrosion is a waste of resources and its reduction is an ECO objective.
Name an example where corrosion is desired.
The roofs of buildings are often covered with copper sheet. A patina is formed when the copper rust. This patina protects the further rusting.
Name 4 reasons for rusting.
Humidity, condensation, gas particles in the air, finger sweat on metal surfaces
What are the consequences of corrosion?
Corrosion degrades the useful properties of materials and structures (strength, appearance and permeability)
What is galvanic corrosion?
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in physical or electrical contact, in the presence of an electrolyte (cathode and anode). The more active metal (anode) corrodes at an accelerated rate than the more noble metal (the cathode).
NAME 4 METHODS TO PREVENT CORROSION.
FIRST THING TO BE DONE IS TO STOP ELECTROCHEMICAL REACTIONS.
FOR GALVANIC CORROSION:
1) PAINTING SURFACE OF METALS THAT ARE EXPOSED TO WATER OR HUMIDITY.
2) REVERSE THE CURRENT ( USING SACRIFICIAL ANODE OR ICCP SYSTEM.
3) BY ISOLATION OF THE TWO METALS OF THE CORROSION CELL.
4) CHEMICAL SURFACE TREATMENT.
EXPLAIN THE PAINTING OF SURFACES.
PERMANENT PROTECTION AGAINST CORROSION WITH LAYERED PAINTINGS OF PRIME COAT AND TOP COAT (EACH WITH DEFINED THICKNESS). COATING IS APPLIED BY SPRAYING OR PAINTING (E.G. BRUSHABLE LIQUID ZINC PAINT: METAL COATING WITH HIGH AMOUNT OF ZINC ENSURES EFFICIENT CORROSION PROTECTION). PROPER PREPARATION OF SURFACE (METHODS CALLED GRINDING OR BLASTING, SUBSTANCES HAVE TO BE REMOVED PRPERLY) IS IMPORTANT (GREASE-FREE, NO DIRT/RUST/SLUG). EXAMPLES:
1) PRIME AND TOP COATING OF BOGIE FRAME OR CAR BODY (ECO4/19,20)
2) ONLY PRIME COATING FOR BOLTED JOINTS OF ROLLERCOASTER
PROBLEMS:
1) PAINT DISAPPEARING ON E.G. EDGES OF BENCHES BECAUSE OF SHARP EDGES WHERE THE PAINT PULLS BACK FROM BECAUSE OF THE PAINT’S SURFACE TENSION, CONSEQUENCE IS CORROSION (4/24)
2) POOR ACCESSIBLE AREAS (FOR PREPARATION AND COATING OF SURFACE)
3) TOO THICK COATING CAUSED BY DIFFICULT ACCESSABILITY CAN CRACK
4) SHARP CORNERS AND NOT DEBURRED EDGES WILL CAUSE DIFFICULTIES TO OBTAIN THE DESIRED LAYER THICKNESS.
EXPLAIN THE METHOD OF THE SACRIFICIAL ANODE
PREVENTING CORROSION BY LESS NOBLE METALLS (ZINC, AL) WHICH ARE WELDED (USING STEEL STRIPS) ON THE PARTS OF THE SURFACES WHICH ARE CONTINUOUSLY EXPOSED TO WATER. THE LESS NOBLE METALS THEN ATTRACT THE CURRENT OF THE STEEL STRIPS BEFORE IT ENTERS THROUGH THE COPPER OF THE SHIP SKIN ODER WATER PIPES. SO ITS THE METALS WHICH ARE DISSOLVING INTO THE WATER, AND NOT THE COPPER.
TO SUMMARIZE: SACRIFICIAL ANODE CATHODIC PROTECTION PREVENTS CORROSION ON A COMPONENT BY FORMING A GALVANIC CELL (CONSISTING OF ADDITIONAL METALLIC ANODE WHICH IS CORRODING IN PREFERENCE TO THE COMPONENT).
GALVANIC CORROSION IS AFFECTED BY TEMPERATURE, HUMIDITY, ANODE SIZE, MATERIAL.
EXPLAIN THE METHOD OF IMPRESSED CURRENT CATHODIC PROTECTION (ICCP)
PREVENTING CORROSION BY POSITIVE CURRENT WHICH IS PASSED IN THE WATER SURROUNDING THE SHIP OR PIPELINE. THUS, NO REACTIONS OCCUR. ANODES ARE FIXED INSIDE THE SHIP’S SURFACE AND MADE OF NON REACTIVE SUBSTANCES WHILE A RECTIFIER (STROMRICHTER) IS CONNECTED TO THE SURFACE STEEL.
EXPLAIN THE METHOD OF CHEMICAL SURFACE TREATMENT USING THE TERM PASSIVATION AND STAINLESS STEEL
PASSIVATION MEANS A MATERIAL BECOMING PASSIVE, I.E. LESS AFFECTED BY ENVIRONMENTAL FACTORS (E.G. WATER). THIS IS REALIZED BY THE GENERATION OF OUTER SHIELDING LAYERS (COATING) OR SPONTANEOUSLY IN NATURE. A LIGHT COAT OF MATERIAL LIKE METAL OXIDE PREVENTS CORROSION.
PASSIVATION CAN ALSO BE USEFUL FOR STAINLESS STEEL, AS THESE ARE NOT COMPLETELY IMPERVIOUS TO RUSTING DESPITE ITS ALLOYING CHROMIUM. A PASSIVATION PROCESS FOR STAINLESS STEEL IS CLEANING WITH HYDROXIDES AND ACIDS WHICH RESTORES THE FILM AND REMOVES DIRT. FOR THE STAINLESS STEEL TO STAY MORE OR LESS CORROSION RESISTANT, IT’S IMPORTANT NOT TO USE TOOLS ON THEM WHICH WERE PREVIOUSLY USED FOR STANDARD STEEL. NORMAL STEEL CAN BE PASSIVATED BY BLUING WHICH GENERATES A COATING PROTECTING AGAINST RUST.
WHAT IS THE DIFFERENCE BETWEEN PURE ALU AND ALU ALLOYS REGARDING CORROSION?
PURE ALU CAN CREATE A PHYSICAL BARRIER TO CORROSION BY OXIDATION (NATURALLY FORMING A THIN SURFACE LAYER OF ALUMINUM OXIDE ON CONTACT WITH OXYGEN IN THE ATMOSPHERE), WHEREAS ALU ALLOYS OFFER LITTLE PROTECTION AGAINST CORROSION (PASSIVATION BY E.G. PRODUCING PURE ALUMINUM OR CHROMATE COATING).
CAN CORROSION BE AVOIDED? HOW CAN IT BE PREVENTED/REDUCED?
IN MANY CASES NOT, IT CAN ONLY BE REDUCED, BECAUSE THE CAUSE OF THE CORROSION (E.G. WATER) OFTEN CAN’T BE REMOVED. CORROSION-RESISTANT MATERIALS CAN’T ALWAYS BE REALIZED (TECHNICAL AND COST REASONS).
UNACCEPTABLE CORROSION HAS TO BE PREVENTED/REDUCED BY A PURPOSEFUL CONCEPT OR DESIGN BY ENGINEERS (E.G. FOR AIRPLANES: BONEYARD IN CALIFORNIA’S MOJAVE DESERT, EXTREMELY LOW HUMIDITY LEADS TO NO CORROSION OF ACTUALLY NON-USED AIPLANES!)
