NBS

Publications

81-2339

A111D1 7 7 ( 5 M ^ Simulated Service Testing for

Corrosion in Solar Heating and

Cooling Systems

NBSIR

NAT'L INST. OF STAND & TECH R.I.C.

A 11 ID 5 A fl 7 2

5

June 1 981

Issued September 1981

Prepared for: U S. Department of Energy

Office of Solar Applications for Buildings

nservation and Renewable Energy

lington, DC 20585

±JJ

. Ibo

i Jol

NATIONAL BURBA

or standards

NBSIR 81-2339 LIBRA RT

/tr SIMULATED SERVICE TESTING FOR

CORROSION IN SOLAR HEATING AND

COOLING SYSTEMS

MAR 4 1983

riot a~cc - - (Za. QtCfOO

, (A

no. F/o 3 • mt

Paul Wencil Brown

John W. Grimes, Jr.

June 1 981

Issued September 1981

Prepared for: U.S. Department of Energy

Office of Solar Applications for Buildings

Conservation and Renewable Energy

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

This study was undertaken to evaluate a proposed ASTM simulated service test

methodology to evaluate corrosion or heat transfer liquid degradation* The

responses of aluminum, copper, and stainless steel to conditions simulating

flow and stagnation in solar collector systems were evaluated. The chemical

stabilities of ethylene and propylene glycol solutions at elevated temperatures

were also examined.

Key Words: corrosion; elevated temperature; heat transfer liquid degradation

kinetics; simulated service test solar collector

iii
TABLE OF CONTENTS

Page

Abstract ................................ . iii

List of Tables v

List of Figures vi

I. Introduction 1

2. Types of Tests 2

3. Defining Design and Operating Parameters as a Basis for Simulated

Service Testing 3

4. Design of a Simulated Solar Loop System - Description

of Apparatus 4

5. Operation of the Loop Systems 8

6. Heat Transfer Liquid Stability 11

7. Simulated Service Test Conditions 19

8. Simulated Service Test Results 21

9. Discussion of Test Results 24

10. Acknowledgement 25

II. References 26

iv
LIST OF TABLES

Table 1. Solar System Design and Operating Parameters

Table 2. Absorber Plate Stagnation Temperatures .....

Table 3. Summary of Glycol Stability Study ..........

Table 4. Summary of Test Conditions .................

Table 5. Summary of Test Results ....................

v
LIST OF FIGURES

Page

Figure 1. Schematic of Loop System 6

Figure 2. Schematic of Loops 7

Figure 3. Time-temperature Profile 9

Figure 4. Electro-mechanical Schematic of the Loop Systems 13

Figure 5. Apparatus Used in Glycol Study 14

Figure 6. Plot of Solution pH vs. Time for Aerated Solutions at

100°C 16

Figure 7. Plot of Solution pH vs. Time for Deaerated Solutions at

100°C 17

Figure 8. Plot of Solution pH vs. Time for Aerated Solutions at

100°C in the Presence of A1 and Cu 18

vi
1. INTRODUCTION

Among the principal concerns relating to the utilization of active solar

systems for space or water heating are those regarding durability. One of

the major factors which affect the durability of a solar heating system is

the interaction between the heat transfer liquid and its containment system.

This interaction may result in changes in the properties of the heat transfer

liquid, corrosion of the metallic components of the absorber plate as well as other parts of the containment system, or both. In addition, these phenomena

show varying degrees of synergism depending on the compositions of the metal(s)

and heat transfer liquid. It is the purpose of this report to describe a test

methodology that was used to evaluate the responses of simulated metallic

containment systems and heat transfer liquids to a variety of operating and

stagnation conditions. This is part of a study to assess the validity of a proposed ASTM accelerated simulated service test methodology.

1
2. TYPES OF TESTS

The test methodologies used in evaluating internal corrosion and heat transfer

liquid degradation in solar systems tend to fall into 3 general categories:

1) Screening tests

2) Simulated service tests

3) Field tests.

Screening tests are usually short-term tests in which a metal coupon is placed

in contact with a heat transfer liquid and the effect of one or more of the

design or operating parameters of a solar systems (usually elevated temperature) is examined. A series of tests of this type have been developed in ASTM

Committee E44 on Solar Energy Conversion [1, 2]. Simulated service tests are generally designed to consider most or all of the system design and operating

parameters. Field tests involve evaluating a system component or subassembly

in service.

2
3c DEFINING DESIGN AND OPERATING PARAMETERS AS A BASIS FOR SIMULATED SERVICE

TESTING

The parameters listed in table 1 are among those of importance in affecting

the corrosion of a metallic containment system and the degradation of a heat

transfer liquid.

The effects of the number of collector cover plates and the use of selective

vs. non-selective absorber surfaces on stagnation temperatures is shown in

table 2 [3].

In systems which drain, such as swimming pool heaters, stagnation conditions

will occur when the absorber is empty except for residual liquid and may

result in the concentration of aggressive anions due to evaporation. In nondraining systems, the temperature of the liquid in the absorber may approach

the temperatures listed in table 2 should stagnation occur. This, in turn,

may cause either oxidative or thermal degradation of the heat transfer

liquid.

The geometry of the flow passages will affect the local flow rates, possibly

resulting in regions of low flow or turbulence and some flow passage geometries may be conducive to pitting or crevice corrosion. Flow passage geometry will also affect the ratio of the area of internal metal surface to

liquid volume.

A collector system may operate at atmospheric pressure under conditions where

oxygen is accessible to the liquid, it may operate at atmospheric pressure

with an inert gas blanket above the resevoir, or it may operate under pressurization. These operating conditions will affect the degree of liquid

aeration.

Dissimilar metal contact is likely in most systems, principally in pumps and

valves.

Heat transfer liquds may be aqueous, such as potable water, swimming pool

water, or water-glycol mixtures, or non-aqueous such as organic or silicone

oils. Heat transfer liquid selection may be based on considerations of toxicity, viscosity which affects pump capacity requirements, chemical stability

or inertness, and heat transfer characteristics which affect thermal efficiency. The heat transfer characteristics of the liquid will, in turn, affect

the operating temperature, the stagnation temperature, and the temperature

differential between the containment system and the heat transfer liquid.

3
4. DESIGN OF A SIMULATED SOLAR LOOP SYSTEM - DESCRIPTION OF APPARATUS

Cognizant of the design and operating parameters outlined in table 1, several

loop systems were constructed to allow evaluation of the effects of continuous

flow and stagnation on the corrosion of containment materials or allow evaluation of the stability of heat transfer liquids. Figure 1 is a schematic of

these loop systems. The loop systems were designed to allow any metallic

containment material or any heat transfer liquid to be tested. In the present

case, 12.7 mm (0.5 inch) o.d. , 9.5 mm (0.375 inch) i.d., 3003 aluminum alloy

tubing, 122 copper alloy tubing, 316 stainless steel tubing were used. With

the exceptions of 316 stainless steel flowmeter heads and soldered joints in

the copper systems, the flow passages of the loop systems were designed to be

constructed entirely of the metal being tested and non-metallic materials.

The overall dimensions of the loops were determined by the internal diameter

of the flow passages. To ensure laminar flow across the samples, the minimum

length between the sample arrays and bends in a loop leg was 40 times the

flow passage diameter.

A typical loop leg is shown in figure 2. Three samples, each 101.6 mm (4

inches) in length, were placed in the center of each leg of the loop. The

edge of each sample was machined to accomodate a Viton* 0-ring. This ensured

electrical isolation between each sample and the remainder of the loop system.

Teflon* retainers were used to align the samples. These were loosely fitted

into aluminum guides. The aluminum guides were interconnected by threaded

rods and held in place by stainless steel set screws. This design was selected because it minimizes the direct contact between the samples and the support system. In addition other sample geometries, for example, a section

cut from an actual absorber plate could be used.

Four additional aluminum guides were placed on each leg as support points for

aluminum sheathing. The sheathing was selected to allow a 3.2 mm (0.125 inch)

gap between its internal diameter and the aluminum guides. The sheathing was attached to the tubing support system by set screws, thus minimizing direct

contact. Heating tape interspaced with four layers of insulating material was uniformly wrapped around the sheathing. The temperatures at which testing was carried out was controlled by variable transformers and monitored by thermocouples placed along each loop. Testing under stagnation conditions at 90°C

indicated that the temperature gradient between the top and bottom of a loop was 2°C thus indicating that relatively uniform heat transfer was achieved.

* Certain trade names and company products are identified in order to

adequately specify the experimental procedure. In no case does such

identification imply recommendation or endorsement by the National Bureau

of Standards, nor does it imply that the products are necessarily the

best available for the purpose.

4
Table 1

Solar System Design and Operating Parameters

Design Parameters Operating Parameters

i) Number of covers; selective

vs. non-selective absorber

surface

i) Operating and stagnation

temperature

ii) System drains down ii) Stagnation, empty or partially full

iii) System does not drain down iii) Stagnation, full

iv) Geometry of liquid flow passages (crevices, areas of low

flow or turbulence, metal surface to liquid volume ratio)

iv) Flow rate

vi) Metal selection (dissimilar

metal contact)

vi) —

vii) Heat transfer liquid selection vii) Operating temperature

viii)° Heat transfer characteristics viii) Variable temperature gradient:

across metal-liquid interface

Table 2*

Absorber Plate Stagnation Temperatures

One Glass Cover

0 . 17oFe203

Two Glass Covers

Non-selective Selective Non-selective Selective

143°C 205°C 170°C 219°C

* T ambient 25°C, Solar radiation 1070 W/m^

5
Figure 1. Schematic showing the various components of the loop systems. The

systems are initially charged with liquid through the priming port [1]

by opening the manual two-way valve [2]. The common flow path [3] is

from the reservoir [4], through the pump [5], to the 3-way solenoid

valve [6]. The primary flow path [7] is across the samples. The flow

path is changed from primary to secondary by activating the 3-way solenoid valve [6] and the secondary path [8] during loop stagnation is

through the flowmeter [9]. The hot plate [10] maintains the liquid

temperature during stagnation. In systems which stagnate above the

boiling point of the liquid, a 2-way solenoid valve [11] placed at the

top of the loop is activated during stagnation. In continuous flow

systems, a manual 3-way valve replaces the 3-way solenoid valve [6].

6
ALUMINUM

SHEATHING

Figure 2. Schematic of a typical loop showing the samples, the sample

support system, and the sheathing support system.

7
5 . OPERATION OF THE LOOP SYSTEMS

Operating and stagnation temperatures will, strongly influence the corrosion

rate of a containment system and the decomposition of a heat transfer liquid.

Therefore, the loop systems were designed to operate in 3 modes to allow an assessment of these effects. The modes of operation were:

1) Continuous flow

2) Flow-stagnation cycles at temperatures below the boiling

temperature of the liquid

3) Flow-stagnation cycles at a flow temperaure below the boiling

temperature of the liquid and at a stagnation temperature

above the boiling temperature of the liquid.

The essential differences between these designs was in the utilization of

solenoid valves (see figure 1). In the systems designed for continuous flow,

a manual 3-way valve was placed between the pump and the loop. Occasionally

this valve was closed to the loop and opened to the flowmeter to observe the

flow rate. In the systems with a stagnation cycle, a 3-way solenoid valve

was placed in a similar location. This valve was connected to a timer-cycler

which allowed it to be activated on any preset cycle. When this valve was activated the flow of liquid to the loop was diverted and a column of liquid

was trapped in one leg while the other leg drained resulting in stagnant full

and stagnant empty conditions, respectively. In the systems which stagnated

at a temperature above the boiling temperature of the liquid, a 2-way solenoid

valve placed at the top of the loop and activated simultaneously with the 3-

way valve. The flow-stagnation cycle used was 4 hours of flow followed by 4

hours of stagnation. The flow temperature used was 90' C while stagnation

temperatures were 90, 120, or 135°C.

Figure 3 shows a time temperature profile typical of a system which stagnates

at 120' C. At time t 0 , heat transfer liquid is flowing through the loop and

the temperature is 90' C. At time tq, the power to the heating tapes is

switched off by deactivating the two variable transformers which control the

flow temperature. The temperature decreases until time t 2 is reached. The

variable transformers which control the stagnation temperature and the solenoid valves which determine the flow path and isolate the stagnant-full loop

leg (valves 6 and 11 in figure 1) are activated at time tq. The temperature

begins to rise to 120 eC. Stagnation continues to time t 3 when the solenoid

valves open, the variable tranformers controlling the stagnation temperature

are deactivated and those controlling the flow temperature are activated.

The temperature drops rapidly as 70'C liquid from the reservoir flows through

the loop, reaching a minimum at time t^. The temperature then rises to 90 'C as flow continues.

An electromechanical schematic of the loop systems is shown in figure 4. The

durations of the flow-stagnation cycles were controlled by a timer-cycler

which consisted of three raicroswitches and three cams driven by a timer motor

geared to 0.25 rph. Microswitch No. 1 controlled the opening and closing of

the three-way solenoid valve, labelled valve #6, which controlled the heat

transfer liquid flow path. Microswitch No. 2 controlled the activation of

8
(Do! BurunodNii.

9

Figure

3.

Time-temperature

profile

of

a

system

which

stagnates

at

120°C.
the heating tapes. This was done by activating two power relays each of

which interfaced between two variable transformers and the heating tapes.

The heating tapes on each leg of the loop were powered independently. It was necessary to design the system in this way in order to maintain the desired

flow and stagnation temperatures on each loop leg. A total of four variable

transformers were used. Two were used to set the flow temperatures while the

other two were used to set the stagnation temperatures. Microswitch No. 3 was used to activate the two-way solenoid valve, labelled valve #11. This

valve was used in systems which stagnated above the boiling point of the

heat transfer liquid.

The cam system activated the three microswitches slightly out of phase. This

along with a time delay relay was used to minimize the temperature excursions

which occurred in going from the flow modes to the stagnant modes (see figure

3). The variable transformers which controlled the stagnation temperatures

were deactivated about 30 minutes before flow commensed. This allowed the

heat transfer liquid to cool to below its boiling point prior to the initiation of flow. The three-way valve was set to open a few minute after two-way

valve opened and to close a few minutes before the two-way valve closed. This

was accomplished by using a time delay relay. Because the power required to

maintain the stagnation temperature was lower than that required to maintain

the flow temperature, the variable transformers which controlled the flow

temperature were set to deactivate and those which controlled the stagnation

temperature set to activate about 15 minutes before stagnation began.

The internal volumes of the loop systems were approximately 390 ml and the

internal surface areas approximately 650 cm^ . The liquid volume to internal

surface area was maintained at approximately 3 by charging each system with

2000 ml of liquid. Two-liter, glass reaction vessels were used as reservoirs.

Hot plates placed under each reservoir were used to maintain the temperature

of the liquid in the reservoirs at 70®C during stagnation cycles. Flow rates were controlled by variable speed pumps and were maintained at 0.64 m/s

(2.5 ft/s).

10
6. HEAT TRANSFER LIQUID STABILITY

Prior to constructing the loop systems, a series of tests were carried out to

examine the stabilities of ethylene glycol and propylene glycol. These tests

were carried out at 100°C in the presence of aluminum and copper under conditions of aeration and deaeration. This was done in order to separate the

effects of thermal and oxidative decomposition of these liquids while bracketing the degree of aeration likely to occur in the loop systems and to examine

changes in the liquids as a function of time as a basis for determining a reasonable test duration.

Reagent grades ethylene and propylene glycol were diluted to 50 volume percent

with ASTM D1384 water [4]*. These solutions were placed in round bottomed

flasks and weighed coupons of 1100 aluminum or 110 copper alloys were placed

in the solutions. The flasks were heated to 100°C and the solutions either

deaerated with high purity N2 or aerated with laboratory air. No provision

was made to remove carbon dioxide in the aeration experiments. Aliquots of

liquid were removed biweekly for analysis and after 16 weeks the tests were

terminated and the coupons reweighed and visually inspected. The experimental

details and the results of this study are presented in table 3.

Figure 5 shows the apparatus used in these experiments. This consisted of a 3-necked, round bottom, flask which contained the test solution. A gas dispersion tube was inserted in one neck, a condenser tube in the second, and

the third neck, which was used for sampling, was stoppered. The temperature

of the solution within the flask was held constant within +1°C by using a double boiler arrangement. The round bottom flask was suspended in a specimen

jar containing a polyglycol heat transfer liquid. The temperature of this

liquid was maintained using a controlled temperature hot plate.

Based on the assumption that the degradation of ethylene and propylene glycol

results in the generation of acidic reaction products, variation in solution

pH with time was used as a measure of the degree of degradation. Under

aeration conditions pH changes may result from both oxidative and thermal

decomposition of the liquid. Further, the presence of metals may in some measure catalyze liquid decomposition. Under conditions of deaeration,

oxidative liquid decomposition is minimized.

Figure 6 shows the variation of solution pH over time for aerated solutions

of ethylene and propylene glycol held at 100°C. This figure indicates that

propylene glycol to show significantly greater stability than ethylene glycol

under aeration conditions when no metal is present. The rapid initial

increase in pH observed in this figure appeared to occur in all of the experiments regardless of the solution composition, the presence of metal, and

regardless of extent of liquid aeration. While the precise reason for the

* ASTM D1384 water contains NaCl, Na2 SC>4 and NaHCC>3 at concentrations of 100

ppm.

11
Table 3

SUMMARY OF GLYCOL STABILITY STUDY

Experimental Conditions

Temperature

Solution Composition

Solution Volume

Metals Used

Nominal Metal Coupon Dimensions

Aeration Conditions

Test Duration

100°C

50% ASTM D1384 water and reagent grade

ethylene glycol or propylene glycol

700 ml

110 Cu or 1100 A1 alloys

2 x 3 x 0.003 inches (50.8 x 76.2 x 0.076 mm)

Laboratory compressed air or purified N 2

16 weeks

Experimental Results

Solution Metal Aeration Condition Appearance after Test

Ethylene glycol Cu air Total dissolution, blue ppt

Ethylene glycol Cu N 2 Light tarnish

Propylene glycol Cu air Virtually total dissolution,

green ppt

Propylene glycol Cu n 2 Light tarnish

Ethylene glycol A1 air White film

Ethylene glycol A1 n2 White film

Propylene glycol A1 air Black film

Propylene glycol Al n2 White film

12
TIMER-CYCLER

Figure 4 Electromechanical schematic of the loop systems
Figure 5. Schematic of the apparatus used to study the degradation of

ethylene and propylene glycol.

14
initial pH increase is unknown, it may be related to the loss of atmospheric

CO 2 from the solutions upon heatingc

The thermal aspect of the chemical stability of these glycols was examined by

maintaining the solutions at 100°C while deaerating with purified nitrogen to

minimize the availability of oxygen. Aluminum and copper foils were immersed

in the solutions to examine whether the presence of these metals affected

degradation. Figure 7 shows the pH variations which occurred under these

experimental conditions. The overall variations in pH were small in comparison to those observed under aeration conditions indicating that the availability of oxygen is a major factor in affecting glycol degradation. Further,

while ethylene glycol solutions appear to be thermally more stable than

propylene glycol solutions, the presence of copper reduces the thermal stabillity of both ethylene and propylene glycol solutions relative to that when

aluminum is present.

The effect of the presence of copper and aluminum on pH under aeration

conditions is shown in figure 8. The presence of copper resulted in rapid

decreases in pH in both the ethylene glycol and propylene glycol solutions.

The ethylene glycol solution in contact with aluminum also showed a substantial decline in pH. However, the change of pH of the propylene glycol solution in contact with aluminum was about the same as that which occurred under

deaeration conditions. Thus, both the presence of copper metal and the availability of oxygen accelerate the degradation of ethylene and propylene glycol

while the affect of the presence of aluminum is minimal. Further, while

ethylene glycol appears thermally slightly more stable than propylene glycpl,

it is significantly less oxidatively stable.

The kinetics of these degradation reactions could not be rigorously expressed

by a kinetic equation of any definite order and in some instances it appears

that the kinetic order of the degradation reaction changes with time. This

is shown in figure 7 where it appears that ethylene glycol degradation

approximates a reaction first order in H+ while that of propylene glycol

approximates a first order reaction after about 7 weeks.

In spite of this, these data do qualitatively indicate the relative oxidation

and thermal stabilities of ethylene and propylene glycol under various

conditions.

15
Hd

16

Figure

6.

Plots

of

solution

pH

vs.

time

for

aerated

solutions

of

ethylene

and

propylene

glycol

at

100°C.
17

Figure

7.

Plots

of

solution

pll

vstime

for

deaerated

solutions

of

ethylene

and

propylene

glycol

in

contact

with

aluminum

and

copper

at

100 C
Hd

18

Figure

8.

Plots

of

solution

pH

vs.

time

for

aerated

solutions

of

ethylene

and

propylene

glycol

in

contact

with

aluminous

and

copper

at

100
7. SIMULATED SERVICE TEST CONDITIONS

A total of seven simulated service tests were carried out. Four were

conducted in aluminum systems, two in copper systems and one in a stainless steel system. A summary of the test conditions used is presented in

table 4.

Based on the results of the heat transfer liquid stability tests, ethylene

glycol based heat transfer liquids were used in the aluminum and copper

systems. Ethylene glycol was selected because it was found to be less stable

than propylene glycol in the presence of oxygen and aluminum at the operating

temperatures of the loops. Some heat transfer liquid degradation was desired

in this instance in order to allow an assessment of the effects of the test

conditions after a reasonable test duration. The heat transfer liquid used

in the aluminum systems was a mixture of 50 volume percent of reagent grade

ethylene glycol and 50 volume percent ASTM D1384 water [4]. Due to the interaction of copper with ethylene glycol observed in the heat tranfer liquid

stability tests, a mixture of 25 volume percent reagent grade ethylene glycol,

25 volume percent buffered and inhibited standard antifreeze solution [5],

and 50 volume percent ASTM D1384 water was used in the copper systems. The

heat tranfer liquid used in the stainless steel system was distilled water

containing 1000 ppm of NaCl.

The flow temperatures were maintained at 90°C and flow rates of 0.64 m/s.

were used in all systems. The stagnation temperatures used were 90°C, 120°C,

or 135°C. The tests were carried out for 112 days (16 weeks) in the aluminum

and stainless steel systems and for 168 days (24 weeks) in the copper systems.

19
Summary

of

Test

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*rH

h-3 r—

rH 3 S'? Sj o o a. m 3 rH

5j 3 o Oh CM AJ

u 3 Oh >6 H 3 3

uh rH rH 3 <r 3 2 3 oo -<r 00 •k N 00 56? 3 00 S'? 2 3 co O T5 S 3 3 co o 3 1 3 rH m 3 CL

Sj 3 r-(

m 3 00 Sj

a rH

a

H 3 a 3 co uh Sj rH rH 5j tH o H 2 3 •H o

AJ oh

a 3 OH 60 AJ

H AJ AJ o 3 32 H AJ 32 3 CD 3 3 o 3 AJ C/3 3 AJ 2 3 <d 3 •H rH

22W <J 3 wa

a + 3 rH t—

0) 3 m

—

CO sz Sj >. ** 3 —>6 -

•H AJ 03

X 3 AJ

m Sj

X 3

S-i UH

O 3 3 Or 3 o 3 3

03 o 03 3, 3 60 T3 0u 3 3

AJ AJ 3 3 rH rH 3 03 tH 3 rH 3

CO cO Oh co 3 3 o ^6 rH 2 T3 Sj 3 « -3 3 3 3 r 32 oo

03

H T3 3 h * rH 3 AJ

S-i 03 •H

O 3 cn 3 3 32O 3 X

0)

O AJ 3 rH OH UD •H > CD rH Oh 3

32

a CO a* IW rH 03 3 O MH rH Sj AJ X 03 •H 3 O rH AJ 32 3 3 o Oh o w 52 r4 H & 00 3 <ti CD H a a s rH 3 rH CM Cl <r rH CM r*. AJ =$*= 3 CD

Oh rH rH pH rH 3 3 —

C/3 < <d < aO CD

pH CD < 3 \

Oh CJ CD O CO

rH o CM 60

rH o CM rH

<d CO rH CO

20
8. SIMULATED SERVICE TEST RESULTS

Copper system number 1 operated under conditions of continuous flow at 90°C

for 24 weeks. The solution pH was 8.20. The initial and final reserve

alkalinity values as measured by ASTM D1121 [6] were 3.89 and 3.56, respectively. The average corrosion rate of the 6 specimens tested in this system

was 0.03 mpy.

Copper system number 2 operated on a cycle of 4 hours of flow at 90°C and 4

hours of stagnant at 120°C. The duration of this test was 24 weeks. The

solution pH was 8.20. The initial and final reserve alkalinity values were

3.89 and 3.05, respectively. The average corrosion rate of the 3 specimens

on the stagnant full side of the loop was 0.08 mpy and that of the three

specimens on the stagnant empty side was 0. 12 mpy.

The stainless steel system often operated on a cycle of 4 hours of flow at

90°C and 4 hours of stagnation at 120°C. In this system both sides of the

loop stagnated empty. The duration of this test was 16 weeks. The initial

solution pH was 7.72 and the final solution pH was 8.57. The corrosion rates

of the stainless steel specimens were negligible.

Aluminum system number 1 operated under conditions of continuous flow at 90°

for 16 weeks. The initial solution pH was 8.38 and the final solution pH was

5.56. The average corrosion rate of the 6 specimens tested in this system

was 0.49 mpy. The presence of pitting was observed to be uniformly distributed among the specimens. The maximum pit depth observed was 1.06 mm. The

average of the 10 deepest pits observed was 0.93 mm.

Aluminum system number 2 operated on a cycle of 4 hours of flow at 90°C and

4 hours of stagnation at 90°C. The duration of this test was 16 weeks. The

initial and final solution pH values were 8.38 and 5.08, respectively. The

average corrosion rate of the 3 specimens on the stagnant full side of the

loop was 0.40 mpy while that of the specimens on the stagnant empty side was 0.38 mpy. Neglegible pitting was observed in these specimens.

Aluminum system number 3 operated on a cycle of 4 hours of flow at 90 °C and

4 hours of stagnation at 120°C. The duration of this test was 16 weeks. The

initial solution pH was 8.38 and the final solution pH was 5.33. The average

corrosion rate of the 3 specimens on the stagnant full side was 0.28 mpy while

that of the specimens on the stagnant empty side was 0.54 mpy. The pits

observed in the specimens on the stagnant empty side were both more numerous

and deeper than those observed on the stagnant full side. Maximum pit depths

on the stagnant empty and stagnant full sides were 0.85 and 0.55 mm, respectively. Similarly, the averages of the 10 deepest pits were 0.72 and 0.46 mm,

respectively.

Aluminum system number 4 operated on a cycle of 4 hours of flow at 90°C and

4 hours of stagnant at 135°C. The duration of this test was 16 weeks. The

initial solution pH was 8.38 and the final solution pH was 5.64. The average

corrosion rate of the 3 specimens on the stagnant empty side was 1.34 mpy.

The average corrosion rate of the 3 specimens on the stagnant full side was

21
0.67 mpy. Minimal pitting was observed in these specimens. Pitting on the

stagnant full side was negligible. Pitting was observed in 2 of the 3 specimens on the stagnant empty side and the maximum pit depth was 0.12 mm. Only

6 pits were observed in total and their average depth was 0.09 mm.

Table 5 summarizes the results presented in this section.

22
Table 5

Summary of Test Results

System

T,

°C

Flow/Stag.

pH

Initial 'Final

Corrosion Rate, mpy

Empty Full

Side Side

Deepest Pit

Empty Full

Side Side

Al #1 90 - 8.35 5.56 0.49 1.06 mm

Al #2 90 90 »c 5.08 0.38 0.40 negl. negl

Al #3 90 120 5.33 0.54 0.28 0.85 0.55

Al #4 90 135 •• 5.64 1.34 0.67 0.12 negl

Reserve Aik.

Cu #1 90 - 3.89 3.56 0.03

cu n 90 120 3.89 3.05 0.12 0.08

pH

S/S 90 120 7.72 8.57 negligible

23
9. DISCUSSION OF TEST RESULTS

The responses of both the aluminum and the copper specimens tested under the

various conditions differed depending on the nature of the test conditions

imposed.

The corrosion rates in both copper systems were low. However, operation on

the flow stagnation cycle (copper system number 2) was more conducive to

corrosion than was continuous flow (copper system number 1). The severity of

corrosion, as measured by mass loss, ranked as follows:

Cu 1 < Cu 2, stagnant full side < Cu 2, stagnant empty side

This may be the result of the effects of temperature in that copper system

number 2 showed the higher corrosion rates. In addition, the corrosion rate

on the stagnant empty side of copper 2 was higher than on the stagnant full

side. This may be the result of the concentration of aggressive species during each stagnation cycle as water evaporated from the heat transfer liquid

remaining after this side of the system has drained.

The initial and final heat transfer liquid reserve alkalinities paralleled the

corrosion behavior in the copper systems. The reserve alkalinity of copper

system number 1 decreased by 8% while that of copper system number 2 decreased

by 22%.

The test carried out using 316 stainless steel specimens was inconclusive. A

slight rise in solution pH was observed. However, mass measurements indicated

negligible loss and pitting was not observed.

The four aluminum systems were operated using an uninhibited, unbuffered heat

transfer liquid and the corrosion rates were correspondingly higher. The

corrosion rates on the stagnant empty sides of the three aluminum systems

which operated on flow-stagnation cycles increased with increasing stagnation

temperature.

The corrosion rates on the stagnation full sides of systems 3 and 4 were one half of the respective corrosion rates on the stagnant empty sides of these

systems. However, the corrosion rate on the stagnant full side of system 2 was comparable to that on the stagnant empty side. System 2 stagnated below

the boiling temperature of the heat transfer liquid while systems 3 and 4

stagnated above the boiling temperature of the heat transfer liquid. The

slightly elevated corrosion rate on the stagnant full side in system 2 may

therefore be related to the degree of aeration of the liquid during stagnation. Similar behavior has been observed for 1100 Al in distilled water [7].

The corrosion rate in aluminum system number 1, which operated under conditions of continuous flow was higher than those in the stagnant full sides

of systems 2 and 3. Only when stagnation full occurred at 135°C did the

corrosion rate exceed that of continuous flow at 90°C.

The nature of the corrosion observed in the aluminum systems also varied

with the test conditions. Pitting corrosion appeared to be the dominant

24
corrosion mechanism in systems 1 and 3.. The pitting factors calculated for

these systems ranged from 200 to 280c Alternatively, negligible pitting was observed in system 2. Similarly, negligible pitting was observed on the

stagnant full side of system 4 while only minimal pitting occurred on the

stagnant empty side of this system. Comparison of mass loss and pitting data

indicate the predominant mode of corrosion in the aluminum systems to change

from pitting corrosion to general corrosion in an inconsistent manner.

25
10. ACKNOWLEDGMENT

The authors wish to thank the members of the materials task group of ASTM

committee E44 who participated in the development of this methodology which

has been approved as ASTM E745-80, Standard Practice for Simulated Service

Testing for Corrosion of Metallic Containment Materials for Use with Heat

Transfer Fluids in Solar Heating and Cooling System. The authors express

their appreciation to Thomas Yates for providing the standard antifreeze.

The authors also wish to acknowledge the Department of Energy, Office of

Solar Applications for Building Applications for supporting this study.

26
11. REFERENCES

1. Evaluation of a Proposed ASTM Standard Guide to Assess the Compatibility

of Metal-Heat Transfer Liquid Pairs in Solar Heating and Cooling Systems,

NBSIR 79-1919, Oct 1979.

2. ASTM E712-80, Standard Practice for Laboratory Screening of Metallic

Containment Materials for Use with Liquids in Solar Heating and Cooling

Systems, 1980 Annual Book of ASTM Standards, Part 41.

3. Solar Energy System - Standards for Cover Plates for Flat Plate

Collectors, NBS Tech. Note 1132, Dec. 1980.

4. ASTM D1384-70, Corrosion Test for engine Coolants in Glassware, 1978

Annual Book of ASTM Standards Part 30.

5. General Motors Engineering Standards, GM6038-M, Automotive Engine

Coolant, January 1977.

7. J. E. Draley and W. E. Ruther, Corrosion _12_, pp. 441t-448t (1956).

6. ASTM D1121-78, Standard Method of Test for Reserve Alkalinity of Engine

Antifreeze, Antirusts, and Coolants, 1978 Annual book of ASTM Standards

Part 30.

27

NBS-114A (rev. 2-ec) U.S. DEPT, OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 81-2559

4. TITLE AND SUBTITLE

SIMULATED SERVICE TESTING FOR CORROSION IN SOLAR HEATING AND COOLING SYSTEMS

5. AUTHOR(S)

Paul W. Brown and John W. Grimes, Jr.

6. PERFORMING ORGANIZATION (If joint or other than N BS, see in struction s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State . ZIP)

Department of Energy

1000 Independence Avenue, S.W.

Washington, DC

10.

SUPPLEMENTARY NOTES

| j Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey , mention it here)

This study was undertaken to evaluate a proposed ASTM simulated service test

methodology to evaluate corrosion or heat transfer liquid degradation. The

responses of aluminum, copper, and stainless steel to conditions simulating

flow and stagnation in solar collector systems were evaluated. The chemical

stabilities of ethylene and propylene glycol solutions at elevated temperature

were also examined.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

corrosion; elevated temperature; heat transfer liquid degradation kinetics;

simulated service test solar collector

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