The present invention is directed to a chemical etching composition for etching metals or metallic alloys. The composition includes a solution of hydrochloric acid, phosphoric acid, ethylene glycol, and an oxidizing agent. The etching composition is particularly useful for etching metal surfaces in preparation for subsequent fluorescent penetrant inspection

Foster, Robert E.

Otousa, Joseph E.

Thomas, Clark S.

English

NASA Technical Reports Server

United States Patent 1191 
O'Tousa et al. 
1731 
1211 
1221 
15 11 
1521 
15 81 
METAL ETCHING COMPOSITION 
Inventors: Joseph E. O'Tous~, Northridge; 
Clark S. Thomas, Woodland Hills; 
Robert E. Foster, Ben Lomond, all of 
Calif. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
Assignee: 
Appl. No.: 587,890 
Filed: Sep. 25, 1990 
Int. C l . 5  .......................... B44C 1/22; C23F 1/00; 
C09K 13/00 
U.S. Cl. .................................... 156/664, 156/656; 
252/79.2; 252/79.4 
Field of Search ....................... 156/656, 664, 665; 
252/79.2, 79.4 
[ i l l  Patent Number: 5,034,093 
[45] Date of Patent: Jul. 23, 1991 
[561 References Cited 
U.S. PATENT DOCUMENTS 
4,944,807 7/1990 Sova ............................... 252/79.4 X 
4,970,015 11/1990 Garcia ................................ 252/79.4 
Primary Examiner-William A. Powell 
Attorney, Agent, or Firm-Robert L. Broad, Jr.; Jerry L. 
Seemann 
1571 ABSTRACT 
The present invention is directed to a chemical etching 
composition for etching metals or metallic alloys. The 
composition includes a solution of hydrochloric acid, 
phosphoric acid, ethylene glycol and an oxidizing 
agent. The etching composition is particularly useful for 
etching metal surfaces in preparation for subsequent 
fluorescent penetrant inspection. 
12 Claims, No Drawings 
https://ntrs.nasa.gov/search.jsp?R=19920016156 2020-03-24T07:13:37+00:00Z
5,034,093 
1 ' 2  
which will greatly affect the etching process and cause 
the drawbacks caused above. 
A clear need exists for an etchant composition which 
can be produced easily and quickly, which is storable, 
The invention described herein was made in the per- 5 stable and has a long shelf life and which, when used 
prior to a penetrant inspection testing method, provides 
a surface acceptable for penetrant inspection without 
inducing additional flaws. 
OBJECTS AND SUMMARY O F  THE 
METAL ETCHING COMPOSITION 
ORIGIN O F  THE INVENTION 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). in 
FIELD O F  THE INVENTION. INVENTION 
The present invention is directed generally to a com- 
position for etching metals or metallic alloys. More 
particularly, the present invention is directed to a metal 
or metallic alIoy etching composition comprising a 
solution of hydrochloric acid, phosphoric acid, ethyl- 
ene glycol and an oxidizing agent. Most specifically, the 
present invention is directed to a composition for etch- 
ing metal or metallic alloys prior to penetrant inspection 
Of the metal or metallic alloys comprising a solution Of 2o 
hYdrochloric acid, Phosphoric acid, ethylene glycol and 
femc chloride. 
BACKGROUND O F  THE INVENTION 
It is an object of the present invention to provide an 
effective etchant composition. 
It is a further object of the present invention to pro- 
vide a composition for etching metal or metallic alloy 
surfaces. 
It is another object of the present invention to pro- 
vide an etchant composition which is storable and has a 
long, stable shelf life. 
It is still another object of the present invention to 
provide an etchant composition suitable for use in con- 
junction with conventional fluorescent penetrant in- 
spection methods. 
In accordance with the present invention, an etching 
Chemical etching, in a metallurgical Sense, Can be 25 composition, including hydrochloric acid, phosphoric 
use ofthis compos~t~on eliminates the rejection ofmetal 
defined as a method for revealing structural details by 
cal agent having a different effect On 
stituents* Many purposes for subjecting a 
face to an etching process are known and 
acid, ethylene glycoI and an oxidizing agent is provided. 
articles based on penetrant inspection testing for arti- 
facts induced by the etching process, Etching with the 
the preferential attack of a metal surface with a chemi- 
con- 
sur- 
30 composition of the present invention provides an practiced, including the inspection of metals for struc- quate surface for penetrant inspection without over- tural surface imperfections. etching the surface. The result is improved interpreta- One test utilized for non-destructive detection of tion accuracy of penetrant artifacts and a decreased minute surface flaws and defects in metals is fluorescent 
metal surface a fluorescent liquid penetrant which en- ity as to whether the penetrant artifacts were etch in- 
ters into surface discontinuities or defects, removing duced* 
the defect location by the emiskion of visible fluorescent exhibits a high Ieve1,of stability after formation. The 
light emitted by the retained penetrant upon exposure to 40 composition of the present invention can therefore be 
ultraviolet light. formulated in a laboratory or other chemical manufac- 
prior to employing this process, it is necessary to turing facility, stored indefinitely and shipped to other 
chemically etch the metallic surface to remove any locations when necessary. This eliminates the inconve- 
metal which, due to processes such machining or niences, inefficiencies and inaccuracies of formulating 
grinding, has smeared over the surface and therefore 45 ofthe etchant immediately Prior to the etching Process. 
DESCRIPTION O F  THE PREFERRED masked possible flaws. These flaws, during subsequent 
EMBODIMENT processing or use of the metal, may propagate and cause premature failure if undetected. The etchant utilized in 
this treatment step must remove the surface metal The composition of the Present invention preferably 
within a prescribed time but must not attack grain 50 comprises ferric chloride, hydrochloric acid, phos- 
boundaries or overetch the surface, thereby making the phoric acid and ethylene glYCOl. In the etching compo- 
surface inadequate for accurate fluorescent penetrant sition, ferric chloride serves as a oxidizing agent, hydro- 
inspection. chloric acid as a corrosive agent, phosphoric acid as a 
A commonly utilized pre-penetrant etchant includes leveling agent and ethylene glycol as a modifier to re- 
a mixture of 50% hydrochloric acid and 50% hydrogen 55 duce ionization. Other oxidizing agents, including cu- 
peroxide; however, this etchant has many serious draw- pric chloride, can be utilized in lieu of or in addition to 
backs. Initially, use of the HCl/H202 etchant often ferric chloride. 
produces etch-induced artifacts, such as pitting of the More specifically, the composition comprises from 
metal hardware surface. Subsequent penetrant inspec- about 65-75 volume percent hydrochloric acid, 20 to 30 
tion and detection of these etch-induced flaws results in 60 volume percent ethylene glycol and 3 to 10 volume 
rejection of the metal hardware and subsequent rework- percent phosphoric acid, with the percentages being 
ing the hardware to remove the flaws. based on the total volume of hydrochloric acid, phos- 
A disadvantage related to the 50% HCl - 50% H202 phoric acid and ethylene glycol. 
etch is the volatile, unstable nature of the etchant com- Femc chloride is present in an amount in the range of 
position itself. Because of this instability, the mixture 65 150-250 grams (based on ferric chloride hexahydrate) 
must be formed immediately prior to use in a metal shop per liter of the hydrochloric acid/phosphoric acid- 
where shop personnel may not accurately prepare the /ethylene glycol admixture. Most preferably, the com- 
mixture. The result is an imperfect concentration ratio position comprises a solution of 71 volume percent 
penetrant inspection. This test includes applying to a 35 number Of rejected surfaces by ambigu- 
any excess penetrant from the surface, and identifying The of the present invention further 
5,034,093 
3 
hydrochloric acid, 23 volume percent ethylene glycol, 6 
volume percent phosphoric acid and 190 grams ferric 
chloride (on a ferric chloride hexahydrate basis) per 1 
liter of solution. 
The hydrochloric acid is preferably reagent grade 
while the phosphoric acid is preferably reagent grade or 
food grade. 
The composition of the present invention can be 
easily and simply formulated by combining the ingredi- 
ents in any appropriate manner. Preferably, the compo- 
sition is formulated by first combining the hydrochloric 
acid with the oxidizing agent. After the oxidizing agent 
is completely dissolved, the phosphoric acid and the 
ethylene glycol liquid components are added while 
stirring the solution so that a homogenous mixture is 
achieved. 
The use of the composition of the present invention 
and its performance in the etching process is shown in 
the following example. 
EXAMPLE 1 
A thin groove was machined in the top face of a 
metallic alloy ring and was filled with INCOLOY @ 
903 weld metal. INCOLOY @ is a registered trademark 
of the International Nickel Company. The IN- 
COLOY @ 903 alloy includes the following alloying 
constituents and nominal percentage values: 
Iron 42.0 
Nickel 38.0 
Cobalt 15.0 
Columbium 3.0 
Titanium 1.4 
Aluminum 0.7 
This overlay was completed using robotic welding. 
The weldment was then low stressed machined in a 
conventional manner. After machining, a section of the 
weldment was wet polished with 600 grit wet-or-dry 
silicon carbide papers. One section of the ring was pene- 
trant inspected in the as-received condition to deter- 
mine a baseline for the evaluation. Approximately one 
inch segments along the ring circumference were 
masked off. 
A pre-penetrant etchant solution having the composi- 
tion defined in Table I was applied to the weldment area 
for periods of time in excess of 90 seconds. The etched 
surface area was examined up to 8 0 ~  using a stereo- 
macroscope. After evaluation, the specimens were pen- 
etrant inspected and evaluated. The process was dupli- 
cated for varied weldment surface and heat treatment 
conditions. The surface conditions tested included as- 
machined, polished with 600 grit wet-or-dry silicon 
carbide paper, and polished to less than one micron 
finish in the laboratory. The physical properties tested 
included aswelded, stress relieved and heat treated and 
aged. 
The results of this testing are provided in Table I. For 
all tested treatment times it is observed that all smeared 
metal is completely removed and the surface is accept- 
able for fluorescent penetrant inspection. 
4 
TABLE I 
SO gms F&l3 
300 ml HCl 
25 ml H3PO4 
100 ml Ethylene Glycol 
903 Treatment time 
OVERLAY ( W  Surface Condition 
AS WELDED 90 Satisfactory 
POLISHED 120 Satisfactory 
W/600 PAPER 
AS WELDED 120 Satisfactory 
10 MACHINED 180 Satisfactory 
32 FINISH 
STRESS 120 Satisfactory 
RELIEVED 180 Satisfactory 
HEAT TREAT 120 Satisfactory 
AND 180 Satisfactory 
AGED 210 Satisfacton, ..? 
13 
While the etchant composition of the present inven- 
tion has been described in detail, it is to be understood 
that various changes and modifications which do not 
depart from the spirit and scope of the present invention 
20 may be made. For example, the composition may utilize 
for etching a variety of surfaces, including but not lim- 
ited to micropolished surfaces, surfaces polished with 
600 silicon carbide paper and machined surfaces having 
a 32 RMS finish. 
1. An etchinq composition comprising a solution of 
hydrochloric acid, phosphoric acid, ethylene glycol and 
an oxidizing agent, said hydrochloric acid being present 
in an amount ranging from about 65 to 75 percent by 
volume. 
2. The etching composition of claim 1 wherein said 
oxidizing agent comprises ferric chloride. 
3. The etching composition of claim 1 wherein said 
ethylene glycol is present in an amount ranging from 
about 20 to about 30 percent by volume. 
4. The etching composition of claim 1 wherein said 
35 phosphoric acid is present in an amount ranging from 
about 3 to about 10 percent by volume. 
5. The etching composition of claim 2 wherein said 
femc chloride is present in said composition in an 
amount of about 150 to about 250 grams, based on ferric 
6. The etching composition of claim 2 comprising 
about 71 percent by volume of hydrochloric acid, about 
23 percent by volume of ethylene glycol, about 6 per- 
cent by volume of phosphoric acid and about 190 grams 
ferric chloride, based on ferric chloride hexahydrate, 
7. A method for etching metallic surfaces comprising 
applying to said metallic surface an etchant composition 
comprising a solution of hydrochloric acid, ethylene 
glycol, phosphoric acid and an oxidizing agent, said 
50 hydrochloric acid being present in an amount ranging 
from about 60 to 75 percent by volume. 
8. The method of claim 7 wherein said oxidizing 
agent comprises ferric chloride. 
9. The method of claim 7 wherein said ethylene gly- 
col is present in an amount ranging from about 20 to 
10. The method of claim 7 wherein said phosphoric 
acid is present in an amount ranging from 3 to about 10 
percent by volume. 
11. The method of claim 8 wherein said ferric chlo- 
60 ride is present in said composition in an amount of about 
150 to 250 grams, based on ferric chloride hexahydrate, 
per liter of solution. 
12. The method of claim 8 wherein said etchant com- 
position comprises about 71 percent by volume of hy- 
drochloric acid, about 23 percent by volume of ethylene 
65 glycol, about 6 percent by volume of phosphoric acid 
and about 190 grams ferric chloride based on ferric 
chloride hexahydrate, per liter of solution. 
25 I claim: 
30 
40 chloride hexahydrate, per liter of solution. 
45 per liter of solution. 
' 5  about 30 percent by volume. 
* * * * *  


Metal etching composition

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