FACTORS AFFECTING CORROSION RATES AND INHIBITOR EFFICIENCY

The following factors have been identified as affecting the corrosion rates and inhibitor efficiency:

• Flow rate and type of flow
• Amount of water
• Presence of oxygen, carbon dioxide and hydrogen sulphide
• Temperature
• Welds
• Pre-existing corrosion        
                                                                                         
In review paper on corrosion inhibitor developments and testing in 2004, Gregg and Ramachandran make the following observations:    
   
 • When water is present, corrosion due to carbon dioxide increases with temperature to a point where precipitation of a corrosion product layer occurs  

• The greater the partial pressure of carbon dioxide, the greater the corrosion rate                                                                  
• Increased liquid velocities increases corrosion rates due to rapid transportation of reactant and product species                        
                                       
• Higher liquid velocities result in greater turbulence that increases wall shear stress. This can increase corrosion due to damage being caused to coatings of inhibitor or corrosion product on the pipe wall.                                                                                                                                                                                                                          
PRE-CORROSION
The effectiveness of corrosion inhibitors on surfaces with pre-existing corrosion would appear to be mixed. Some reviewers have found that some inhibitors were able to penetrate deep into rusted layers (Kowata and Takahashi while some have even found an improved inhibitor performance on pre-corroded surfaces. Others have found negligible effect or a negative effect investigated the effect of precorrosion on the effectiveness of corrosion inhibitors. They performed laboratory corrosion tests on carbon steel specimens using the following conditions; 20-50 °C, pH 5, 1 bar CO2 and 1-3 w% NaCl. The specimens were allowed to corrode for up to 18 days in the medium prior to the inhibitor addition.
The following conclusions were drawn from the research:    

• Inhibitor performances were, in general, impaired after long period of precorrosion under the given conditions
                                                     
• Poor inhibition resulted in localised corrosion attacks with deep spherical pits

• The detrimental effect of precorrosion is co-determined by the steel properties and the inhibitor composition. The precorrosion effect seems to be related to the presence of a cementite layer at the steel surface              
                         
• The results showed that the problem could be overcome with careful selection of inhibitors.

Therefore, when choosing inhibitors, laboratory tests should be performed on steels in a condition likely to represent those encountered during service.                                                                                                                                                                                                              
Source:http://www.hse.gov.uk/research/rrpdf/rr1023.pdf