NE EFH 650 Design Guide for Corrosion Control Pgs6-34a-f

7/29/2019 NE EFH 650 Design Guide for Corrosion Control Pgs6-34a-f

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Chapter 6 Structures Part 650Engineering Field Handbook

NEBRASKA AMENDMENT (Formerly EFM)

(Engineering Field Manual Notice NB-21, March 1979) NE 6-34a

DESIGN INFORMATION ANDGUIDE FOR CORROSION

CONTROL OF STEEL USED FOR

UNDERGROUND INSTALLATIONS

(a) Introduction

The following discussions and outlineddesign information are recommended fordesign of systems of corrosion control onsteel pipes installed in soils encountered inNebraska.

The discussion and design procedureoutlined herein are aimed primarily atcorrugated metal pipe, since we have very

few welded or rolled steel pipe installationsin Nebraska for any purpose except wells.

Standard analysis and design procedures,as recommended by Soil ConservationService Design Note 12, Harco's notes onmagnesium anode design, Peabody, andothers, are followed. However, someformulas are modified slightly to make thedesign more compatible to observed resultsof installations on over 100 systems ondams in Nebraska. While the variations

were noted previously, the documentationfor adjusting the formulas for current needswas primarily based on 44 sites selected asrepresentative because they were morethan 5 years old and in soils with resistivitiesranging from 800 ohms to over 2,000 ohms.

(b) Defining The ElectrolyticCorrosion Problem

The three main recognized factors ofcorrosion of underground installations are:

1) soil acidity, 2) soil resistivity, and 3) strayor induced currents. These may besummarized as follows:

(1) The pH of Nebraska soils is not a majorfactor, since the soils have a pH of 6 orhigher with most near the neutral of 7.0. Afew soils have pH's high enough thatprotection by deposition of salts isoccurring.

(2) The resistivity of the soil (ohm/cm) is thebest indicator of the potential for corrosionof a CMP installed in moist to saturatedconditions.

The relative effect of soil resistivity can befurther defined by ranges of resistivity as:

Over 4,000-ohm soil has little or noadverse effect on zinc-coated CMPwith life expectancy of over 30years.

In 4,000-ohm to 2,500-ohm soils,zinc-coated CMP normally will last25 years or longer.

Asphalt coating, or the 'plastic'

coating, in addition to zinc, willextend the life 10 years or more.Cathodic protection should beconsidered for useful life beyond 30to 40 years when a CMP is installedin soils in this resistivity range.

In 2,500-ohm to 1,500-ohm soils,CMP will require a coating inaddition to the zinc coating for a 20-to 25-year life expectancy.

Magnesium anodes will be installedat the time of construction toextend life beyond 20 to 25years.

Soils below 1,500 ohms' resistivityare quite corrosive; therefore, bothadditional coating (either asphalt or'plastic') and anodes will benecessary to assure a life beyond 5or 10 years for the zinc-coatedCMP.

(3) The induced current problems areminimal due to the location of SoilConservation Service structures. Whenthey exist, the design will require specialanalysis outside the scope of this guide.


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NE 6-34b (Engineering Field Manual Notice NB-21, March 1979)

(c) Corrosion Prevention

Corrugated metal pipe protection requires

consideration of several variables. Some ofthese are: 1) the life desired from theworks, 2) the corrosive potential of the soilsin terms of resistivity and acidity, and 3)costs of installation and replacement of thepipe with or without protection.

(d) Coating

Coating used on CMP is basically threetypes: 1) zinc-coated (2 oz/ft2); 2) zinc-coated, asphalt-dipped or asbestos-

impregnated, zinc-asphalt dipped1

; and 3)zinc-coated, pitch resin, or polymer-coated.

All CMP approved for installation inNebraska will have 2 ounces of zinccoating. The zinc, in addition to protectingthe steel from direct contact withsurrounding soil, acts as a sacrificial anodeto protect the steel. The asphalt coatingprovides a further separation of pipe andsoil along with some limited dielectricproperties. The pitch resin or polymercoating provides the separation barrier

along with fair to good dielectric properties.

(e) Design Analysis

Once the determination has been made,based on soil resistivity, design life andother factors, that cathodic protection isneeded, the design follows three basicsteps.

The first step for design of any cathodicprotection system is to estimate the amountof current needed. The basic formula, asmodified based on field tests, is:

1While asbestos-impregnated, asphalt-coated pipe doesretain the asphalt coating better, there is little difference inthe corrosion resistance between this and plain asphalt

coating.

3.0

1000Re

=

RE

CAItma

where: maIt = total current required to

protect the metal

C = constant for quality of pipecoating over the zinc plating whereC = 60 for coal tar, pitch resin, orequals for a Class "B" coating; C =90 for polymer coating (10 mil); C -120 for asphalt coating

A = area of metal to be protected

Re = field-measured resistivity of thesoils in which the pipe is or will beplaced

The variables of type or quality of coatingand the soils' resistivity for the aboveformula have been reduced to the graphson Figure 1 except for the variable "A". Sothe formula now becomes:

AXfromFigaItma )1.(Im=

The second step of designing a cathodicprotection system, using magnesiumanodes, is to determine size and number ofanodes needed.

The output of the magnesium anode is afunction of 1) anode length and diameter, 2)resistivity of the soil around the anode, 3)quality of the pipe coating, 4) potential of thepipe to soil (as measured by the CuCuSO4half-cell), and 5) the number of anodes inparallel.

The chart of anode output versus soilresistivity (Fig. 2) uses the formula:

Re

AdjPsfKIanode

***=

where Ianode = minimum capability of theanode in milliamps

K = a constant for a magnesium alloyof Grade III or better; (adding a50% safety factor and allowing for


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(Engineering Field Manual Notice NB-21, March 1979) NE 6-34c

pipe to soil plus circuit resistance,use K = 60,000)

f = factor for anode length and

diameter; (use '1.0' for 17 lb, '1.06'for 32 lb, '0.71' for 9 lb, '0.60' for 5lb anodes)

Ps = correction for level of pipe to soilpotential; (use -0.90 volts as leveldesired and Ps = 0.93)

Adj = adjustment for the number inparallel; (using a factor of 0.95 willresult in less than 5% error for 1, 2,or 3 anodes in parallel on10 ft ormore spacing)

The total output of an anode, then, is thenumber of anodes times the output peranode. A recommended installation for aprincipal spillway in a structure would betwo anode beds, one near each end of thepipe, each with its own junction box. One tofour separate beds, depending on needs, isacceptable design. Due to maintenanceproblems, especially with undergroundconnections, it is desirable to limit thenumber of anodes per bed to three or lessand use more beds.

The third step of the design is the check ofthe design life. The formula

maIt

WtY

47=

is used, where

Y = years of anticipated life

Wt = total weight of magnesium anodes

Itma = designed current needed

47 = 116 theoretical amp yrs/lbmagnesium x 50% efficiency x80% utilization factor

The nomograph of Figure 3 is based on thisformula; however, it is important toremember that-the mass of an anode, ascompared to the area, also affects theuseful life of an anode. The utilization factorcan be less than 20% with time as an anode

becomes passive or dissipated by reactionwith the soil. The larger anodes will functionlonger due to their bulk factor. Themaximum that can be expected from an

anode bed, therefore, is limited by theindividual anode size as well as the totalpounds of magnesium in the bed. Thenormal maximum life expectancy should be:for 5-lb anodes, 12 years; for 9-lb anodes,15 years; 27-lb anodes, 25 years; 32-lbanodes, 35 years.

Example:

Re = Soil resistivity of 1200 ohm/cmfrom the 4 pin field tests

(Since the soil resistivity is lessthan 1500 ohms, plan touse 10 mil PVC coating)

A = Area of 10' x 42" riser + 100' x30" barrel + 2 6'x6' diaphragms(one side) = 967 ft2

Design

life = 25 years

From Figure 1, at Re = 1200 ohms and C =90 for polyvinyl-coated pipe, Ima/ft2 = 0.07.Total Itma needed = 0.07 ma/ft

2 x 962 ft2 =67 milliamps.

From Figure 2, select the anodecombination that will produce the currentneed. For this example, either three 9-lb,two 17-lb, or two 32-lb anodes will providethe current.

From Figure 3, the three 9-lb anodes willneed to be replaced before the design life of25 years; the two 17-lb anodes will providethe 25-year life with resistors to controlcurrent flow; the two 32-lb anodes would

provide protection to 40 years.

The two 17-lb anodes appear to be the bestcombination if the system will be monitoredregularly and resistance added as neededto maintain a current usage below 70 ma. Ifmonitoring may be irregular, the two 32-lbanodes would be the better choice.


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NE 6-34d (Engineering Field Manual Notice NB-21, March 1979)


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(Engineering Field Manual Notice NB-21, March 1979) NE 6-34e


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NE 6-34f (Engineering Field Manual Notice NB-21, March 1979)

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NE EFH 650 Design Guide for Corrosion Control Pgs6-34a-f