Iron is found in an oxidized state and is mined from the ground as an iron ore.

Size: px

Start display at page:

Download "Iron is found in an oxidized state and is mined from the ground as an iron ore."

Benjamin Howard
6 years ago
Views:

Finally, it is rolled in a pipe mill or rod mill to form pipe, rebar or structural shapes. Corrosion occurs when iron contacts an electrolyte such as earth or concrete.

1 Iron is found in an oxidized state and is mined from the ground as an iron ore. Energy is used to convert this iron oxide into steel. Ore is loaded into a blast furnace where heat energy removes the oxygen producing elemental iron. Finally, it is rolled in a pipe mill or rod mill to form pipe, rebar or structural shapes. Corrosion occurs when iron contacts an electrolyte such as earth or concrete. In that corrosion process, energy is lost and the iron combines with oxygen to form iron oxide. This corrosion product, rust, is identical to the iron oxide mined from the ground. Iron is considered thermodynamically unstable; given the right environment, iron will readily corrode to return to its lower energy state, as rust.

2 Corrosion can be defined as the deterioration of metal due to interaction with the environment. Corrosion is a natural phenomenon that is expected to occur. Metals as high energy materials exist because heat energy was added to natural iron ores during the smelting process. Environmental contact constantly attacks these high energy materials and breaks them back down to the natural elements from which they were derived.

3 The anode and the cathode can be on different metals or on the same metal as shown:

4 For many metals, the rate of corrosion increases appreciably below a ph of about 4. Between 4 and 8 corrosion rate is fairly independent of ph. Above 8, the environment becomes passive and corrosion rates tend to decrease.

5 Corrosion cells may form because of differences in the electrolyte. For example, when a single metal structure spans an electrolyte made up of different types of soils, different chemical substances, different concentrations of the same substance, or temperature variations, the structure may experience voltage differences.

6 Potential Difference Between Anode and Cathode (Galvanic Series) Circuit resistance Resistivity of the Electrolyte Chemical Activity

7 Example connecting zinc with carbon will produce a corrosion cell with a potential of about 1.4 volts. METAL VOLTS (CSE) Commercially Pure Magnesium Magnesium Alloy Zinc Aluminum Alloy Commercially Pure Aluminum Mild Steel (clean & shiny) to Mild Steel (rusted) to Cast Iron (not graphitized) Lead Mild Steel in Concrete Copper, Brass, Bronze High Silicon Cast Iron Carbon, Graphite, Coke +0.30

8 Circuit resistance includes the following: Resistance of the anode Resistance of the cathode Resistance of the electrolyte Resistance of the metallic path Increasing the resistance will reduce the corrosion rate.

9 Soils High resistivity soils reduce the corrosion rate, while low resistivity soils increase the corrosion rate. CLASSIFICATION ELECTROLYTE RESISTIVITY (ohm-cm) ANTICIPATED CORROSIVITY Low Resistance 50 to 2,000 Severe Medium 2,000 to 10,000 Moderate High 10,000 to 30,000 Mild Very High Above 30,000 Increasingly Less

11 Passive (less corrosive) Environment High ph (neutral or basic) Low Moisture Content Lack of Salts High Resistivity Low Temperature Homogenous Environment

12 Uniform or near uniform (slow) Localized (moderate) Pitting (can be very rapid)

13 Uniform or near uniform - Corrosion attacks all areas of the metal at the same or similar rate typically by atmospheric contact. Weathering steel alloy.

14 Localized - Some areas of metal corrode at different rates due to heterogeneities in the metal or environment. This type of attack can approach pitting.

15 Pitting - Very highly localized attack resulting in small pits that may quickly penetrate to perforation.

16 Reference electrodes, or half-cells, are important devices that permit measuring the potential of a metal exposed to an electrolyte. An example is a structure-to-soil potential measurement. Structure-to-soil potentials are measured in reference to an electrode.

17 Saturated CSE Reference

18 Over the center line of the buried structure.

19 Piping Underground or submerged steel, cast iron, aluminum, and pre-stressed concrete cylinder pipelines.

20 Buried Tanks Underground Storage Tanks (UST) and piping

21 Aboveground Storage Tanks Exterior tank bottoms (both primary and secondary) of Aboveground Storage Tanks (AST).

22 Foundation piles Piling

23 Concrete Concrete bridge deck reinforcement and substructures

24 Corrosion can occur at the steel/concrete interface, resulting in spalling. Reinforced concrete is susceptible to chlorides when used in marine environments. Sulfate exposure physically damages concrete.

26 Galvanic (or sacrificial) cathodic protection makes practical use of dissimilar metal corrosion. It requires a substantial potential difference, or driving voltage, between a galvanic anode and the structure to be protected. The galvanic anode is connected to the structure it is protecting, either directly or through a test station so it can be monitored.

27 There are several metals commonly used as galvanic anodes: Aluminum Magnesium Zinc.

29 Typically most effective with electrically isolated and wellcoated structures. No external power source required but limited driving potential (driving potential based upon the galvanic series). Limited output makes it ineffective when trying to protect large surfaces. Requires a low resistivity electrolyte to function well.

30 IM PRESSED CURRENT SY STEM Anode Groundbed Rectifier AC Power Supply Pos itiv e Cable Negativ e Cable Pipeline

31 Graphite Anodes Conductive Polymer High-Silicon Chromium-Bearing Cast Iron Lead-silver Mixed-metal oxide (MMO) Platinum Scrap iron or steel Metallized titanium Thermal sprayed zinc and aluminum Magnetite Aluminum

32 Deep anode installations are those where anodes are installed at the bottom of a drilled hole. Deep anodes are at least m (50 ft) deep

A horizontal groundbed is installed similar to a vertical installation. The anode installation details depend upon the spacing of the anodes.

33 A horizontal groundbed is installed similar to a vertical installation. The anode installation details depend upon the spacing of the anodes. If the spacing is more than 10 feet, it is usually best to excavate a separate hole for each anode. With closer spacing, it is usually more economical to excavate one long trench for both the header cable and the anodes.

35 Potential measurement test stations are used to monitor the effectiveness of cathodic protection, check for stray current effects, and, on unprotected or partially protected pipelines, to locate areas of active corrosion.

38 Test station

40 CIS/DCVG

42 CIS

46 CIS

49 -

Introduction. Jeremy Hailey, P.E.

Introduction. Jeremy Hailey, P.E. Introduction Jeremy Hailey, P.E. Engineer with 16 years experience Started NW 10 years ago Focus on corrosion control system designs, troubleshooting, and some installation work Conduct training seminars