1. WHAT CONDITION IS ESSENTIAL FOR GALVANIC (BIMETALLIC) CORROSION TO OCCUR?
IN ORDER FOR GALVANIC CORROSION TO OCCUR IS ESSENTIAL TO HAVE AN ELECTRICALLY CONTACT BETWEEN TWO METALS WITH DIFFERENT POTENTIALS IN AN ELECTROLYTE.GENERALLY FOR THIS TYPE OF CORROSION TO OCCUR, THE FOLLOWING BASIC CONDITIONS ARE NECESSARY:OXYGEN; ELECTROLYTE; METALS WITH DIFFERING POTENTIALS.
2. HOW DO WE RECOGNISE TURBULENCE CORROSION?
THIS TYPE OF CORROSION (TURBULENT LIQUID FLOW WHICH REMOVES THE OXIDE FILM) OFTEN APPEARS INSIDE COPPER PIPES. IT HAS A CHARACTERISTIC APPEARANCE LIKE SHARPLY PITS, GROUPED OR SINGULAR, WHICH ARE UNDERCUT ON THAT SIDE FACING THE LIQUID FLOW DIRECTION.FOR BRASS PIPES THIS PITTS ARE HORSESHOE SHAPED.
3. WHEN WE USE THE TERM DEZINCIFICATION OF BRASS, WHAT FORM OF CORROSION DO WE REFER TO?
THIS TYPE OF CORROSION IS NAMED SELECTIVE/DE-ALLOYING CORROSION. THE IGNOBLE METAL IS ATTACKED AND DISOLVED.IN CASE OF BRASS THE ZINC GOES IN DISSOLUTION AND COPPER REMAINS AS A POROUS LAYER WITHOUT MECHANICAL STRENGHT.
4. GRAPHITATION IS A SPECIAL FORM OF BIMETALLIC CORROSION ON CAST IRON, AND EITHER THE IRON OR THE GRAPHITE IS CONSUMED THROUGH DISSOLUTION. WHICH OF THE TWO SUBSTANCES WILL GO INTO DISSOLUTION?
THE IRON WILL SACRIFICE HIMSELF FOR THE GRAPHITE.
5. MOST COATINGS FUNCTION BY THE BARRIER PRINCIPLE. AGAINST WHAT IS THE COATING A BARRIER?
THIS FUNCTIONING PRINCIPLE IS TO PRECLUDE BUT NOT FULLY THE INGRESS OF WATER OR OXYGEN TO THE UNDERLYING STEEL SUBSTRATE
6. DOES A COATING HEAVILY PIGMENTED WITH ZINC PHOSPHATE PROVIDE CORROSION PROTECTION BY THE GALVANIC PRINCIPLE?
IF YOU THINK NOT, BY WHAT PRINCIPLE DOES SUCH A COATING PROVIDE CORROSION PROTECTION?
NO, DUE TO THE FACT THAT THE ZINC PHOSPHATE IS ACTING LIKE AN INHIBITOR (REACTION BETWEEN IRON IONS AND INHIBITOR) AND BARRIER EFFECT IN ADDITION. BY GALVANIC PRINCIPLE (PREVENTION OF IRON IONS FORMING) ARE ACTING ONLY THE ZINC PIGMENTATED PAINTS LIKE ZINC ALKALI OR ZINC ETHYL SILICATES.
[ 此帖被hiatc在2011-08-01 15:49重新编辑 ]
