Type of events and consequences of accidents due to corrosion in Refinery

Because of the volume of flammable and explosive substances typically present in refineries,

scenarios tend to include fires and explosions with potentially high consequences if not adequately

controlled.  In particular, production of hydrocarbon products leads to a high presence of flammable

compounds onsite.  Not surprisingly, therefore, nearly 80% of the events studied involved a fire or

explosion (see Figure 4 below).  In addition, a significant amount of toxic substances may be present

such that refineries are also exposed to the risk of potential toxic releases.  Many crude oils contain a

significant percentage of hydrogen sulphide that is eventually separated from the crude and usually

processed to produce sulphur for the marketplace.  Other processes require the presence of sulphuric

acid or hydrofluoric acid (for alkylation) or ammonia (to remove nitrogen from the crude feedstock).

In fact, over one third of the refinery accident events involving corrosion have also generated toxic

releases.  Toxic releases to the soil were slightly higher in relation to toxic releases to water and air,

probably resulting from a number of accidents stemming from tank and underground pipe failures

included in the database.

Releases were most often hydrogen and hydrocarbon compounds including process gases, naphtha,

crude oil and various types of fuels. (See Figure 5 on the next page).  The largest release was

estimated to be around 100,000 tonnes of crude oil followed by 50,000 tonnes of fuel.  Hydrogen

sulphide was the toxic gas released more often than any other (16 cases).  Fewer than 10% of

accidents involved releases of other toxic gases such as hydrogen fluoride, carbon monoxide and

sulphur dioxide. The highest (known) release of a substance toxic to human health was 15 tonnes of

furfural, followed by 1 tonne of sulphur dioxide.

Source:https://ec.europa.eu/jrc/sites/default/files/lbna26331enn.pdf

Corrosion as a major hazard concern for the petroleum refinery industry

Corrosion does not stand for a single phenomenon but is a generalized term to cover a destructive attack on a metal as a result of either a chemical or electrochemical reaction between the metal and various elements present in the environment.  For instance, iron is converted into various oxides or hydroxides when reacting with the oxygen present in air/water, when in contact with a more noble metal such as tin or when exposed to certain bacteria. 

The international standard defines corrosion more specifically as a “physicochemical interaction between a metal and its environment which results in changes of the properties of the metal and which may often lead to impairment of the function of the metal, the environment, or the technical system of which these form a part.”  According to other authors, corrosion derives from “the natural tendency of materials to return to their most thermodynamically stable state.”

Table 3 below identifies four broad categories of refinery elements that can contribute to corrosion risk. Corrosion of a metal occurs either by the action of specific substances or by the conjoint action of specific substances and mechanical stresses.  Depending upon environmental conditions, corrosion can occur in various forms such as stress corrosion, pitting corrosion, embrittlement and cracking.   The particular type of corrosion occurring in a specific component can often be difficult to classify.  

For example, several forms of corrosion (e.g., galvanic corrosion, pitting corrosion, hydrogen embrittlement, stress sulphide corrosion cracking) are characterized by the type of mechanical force to which the metal component is exposed.  It is not within the scope of this work to address in depth either corrosion electrochemistry or the identification of different forms of corrosion.  The basics of corrosion mechanisms are described as a basis for understanding the conditions that make corrosion risks highly relevant for refinery operations and more specifically to provide some insight into the underlying causes of the corrosion events leading to the accidents analysed in this report.  Also, corrosion of certain metals (e.g. aluminium) enhances their corrosion resistance, but in this work corrosion is assumed to be solely an undesirable phenomenon.

Source:https://ec.europa.eu/jrc/sites/default/files/lbna26331enn.pdf

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

What is Cathodic Protection?

Cathodic protection is one option for protecting an underground storage tank (UST) from corrosion. There are two types of systems for cathodic protection:

•  Sacrificial anode
•  Impressed current

Sacrificial anodes can be attached to a coated¹ steel UST for corrosion protection. Sacrificial anodes are pieces of metal more electrically active than the steel UST. 

Because these anodes are more active, the corrosive current will exit from them rather than the UST. Thus, the UST is protected while the attached anode is sacrificed. Depleted anodes must be replaced for continued corrosion protection of the UST.

An impressed current system uses a rectifier to convert alternating current to direct current (see below, right). This current is sent through an insulated wire to the anodes, which are special metal bars buried in the soil near the UST. The current then flows through the soil to the UST system and returns to the rectifier through an insulated wire attached to the UST. The UST system is protected because the current going to the UST system overcomes the corrosion-causing current normally flowing away from it.

Regulations require that the cathodic protection systems installed at UST sites (field-installed) be designed by a corrosion expert.

The system must be tested by a qualified cathodic protection tester within six months of installation and at least every three years thereafter. In addition, cathodic protection systems must be tested within six months of any repair to any cathodically protected UST system. You will need to keep the results of the last two tests to prove that the cathodic protection is working. In addition, you must inspect an impressed current system every 60 days to verify that the system is operating. Keep results of your last three 60-day inspections to prove that the impressed current system is on and operating properly.

Source:http://www.epa.gov/oust/ustsystm/cathodic.htm

Corrosion as a major hazard concern for the petroleum refinery industry

Corrosion does not stand for a single phenomenon but is a generalized term to cover a destructive attack on a metal as a result of either a chemical or electrochemical reaction between the metal and various elements present in the environment. 

For instance, iron is converted into various oxides or hydroxides when reacting with the oxygen present in air/water, when in contact with a more noble metal such as tin or when exposed to certain bacteria.  The international standard defines corrosion more specifically as a “physicochemical interaction between a metal and its environment which results in changes of the properties of the metal and which may often lead to impairment of the function of the metal, the environment, or the technical system of which these form a part.” According to other authors, corrosion derives from “the natural tendency of materials to return to their most thermo dynamically stable state.” 

Table 3 below identifies four broad categories of refinery elements that can contribute to corrosion risk.

Corrosion of a metal occurs either by the action of specific substances or by the conjoint action of specific substances and mechanical stresses.  Depending corrosion (e.g., galvanic corrosion, pitting corrosion, hydrogen embrittlement, stress sulphide corrosion cracking) are characterized by the type of mechanical force to which the metal component is exposed.  It is not within the scope of this work to address in depth either corrosion electrochemistry or the identification of different forms of corrosion. 

The basics of corrosion mechanisms are described as a basis for understanding the conditions that make corrosion risks highly relevant for refinery operations and more specifically to provide some insight into the underlying causes of the corrosion events leading to the accidents analysed in this report.  Also, corrosion of certain metals (e.g. aluminium) enhances their corrosion resistance, but in this work corrosion is assumed to be solely an undesirable phenomenon.

Source:https://ec.europa.eu/jrc/sites/default/files/lbna26331enn.pdf