Inhibitor of the corrosion of a metal material such as steel

The invention relates to a corrosion inhibitor for a metal material such as steel, of the type consisting of an aqueous solution containing an alkali metal nitrite, wherein said solution contains 0.01 to 0.3 mol/l of an alkali metal nitrite, 0.01 to 0.3 mol/l of an alkali metal phosphate, and phosphoric acid in a proportion of a quantity sufficient to maintain the pH of said solution, measured at ambient temperature, between 6.9 and 7.2.

FILED OF THE INVENTION 
The invention relates to the field of corrosion inhibitors with Which the 
surface of metals, especially of steel, is coated before they are stored. 
PRIOR ART 
Various solutions containing corrosion inhibitors are employed by steel 
manufacturers for coating, together with oils, the surface of steel sheets 
after they are rolled and before they are coiled and sent off to the 
customer who has ordered them. These corrosion inhibitors are intended to 
passivate the surface of the product (in the case of passivating 
inhibitors based, for example, on nitrites), by forming a passive film 
thereon or to form compounds which precipitate at the surface of the 
product and form a protective film thereon (in the case of nonpassivating 
inhibitors based, for example, on phosphates), or to cause a 
neutralization or an alkalifying of the medium next to the surface and an 
adsorption (in the case of amine-based organic inhibitors). These 
inhibitors can also serve as lubricants during the subsequent cold working 
operations on the sheet. 
This protection of the sheets using such inhibitors is particularly 
necessary in the case of steel sheets which have been coated by 
electrodeposition on only one of their faces. Firstly, during the various 
stages linked with this treatment, the sheet is in contact with various 
liquids which may be corrosive to the uncoated face if they are not 
subsequently completely removed: degreasing and cleaning solutions, 
electrolyte, rinsing fluids etc. In addition, the atmosphere of the 
electroplating line is charged with electrolyte salts and acid. All the 
conditions are therefore combined for requiring a treatment of the 
uncoated face of the sheet with a corrosion inhibitor in order to avoid 
its pitting while the reel is being stored. 
Inhibitors containing both sodium nitrite NaNO.sub.2 and azoles have proved 
their effectiveness for the protection of steel sheets. However, they 
cannot be employed for sheets only one face of which is coated and 
comprises, for example, a deposit of a zinc-nickel alloy, because during 
coiling the contact between this coating and the inhibitor deposited on 
the bare face of the adjoining turn causes the appearance of stains on the 
zinc-nickel layer. However, they do not run the risk of generating 
nitrosamines during any possible subsequent reheating of the sheet, in 
contrast to amines. Since nitrosamines are known for their carcinogenic 
properties, such a possibility of formation must be avoided. 
The aim of the invention is to propose a corrosion inhibitor that can be 
employed on steel sheets coated on only one of their faces, without this 
inhibitor's damaging the coated face and generating toxic compounds during 
a subsequent heating.

SUMMARY OF THE INVENTION 
To this end, the subject of the invention is a corrosion inhibitor for a 
metal material such as steel, of the type consisting of an aqueous 
solution containing an alkali metal nitrite, wherein said solution 
contains 0.01 to 0.3 mol/l of an alkali metal nitrite, 0.01 to 0.3 mol/l 
of an alkali metal phosphate, and phosphoric acid in a proportion of a 
quantity sufficient to maintain the pH of said solution, measured at 
ambient temperature, between 6.9 and 7.2. 
As will be understood, the invention consists in jointly adding an alkali 
metal phosphate and phosphoric acid to the known sodium-nitrite-based 
inhibitors, in well-determined proportions, so that a neutral solution is 
obtained which is noncorrosive to the coated face of the sheets with one 
zinc-nickel face. 
The inventors have found that the appearance of stains on the zinc-nickel 
coating of the coated face of the sheets whose bare face is protected by 
sodium nitrite inhibitors was due to the pronounced basicity and to the 
absence of a buffer medium for these inhibitors. Their pH is approximately 
9 to 11, and this causes an alkaline corrosion of the coating by the 
formation of zincates. The idea underlying the invention is that it is 
possible to avoid this alkaline corrosion by buffering the inhibitor at a 
neutral pH compatible with passivation of the iron. This can be carried 
out by adding a buffer salt to the sodium nitrite. A mixture of sodium 
phosphate Na.sub.3 PO.sub.4 and of phosphoric acid H.sub.3 PO.sub.4 is 
particularly indicated (it being impossible for the sodium to be replaced 
with another alkali metal such as potassium). In fact, the nitrite ions 
and the phosphate ions have synergistic effects in their anticorrosion 
action, the former acting as neutralizing inhibitors and the latter as 
passivating inhibitors. 
The fact of providing a buffer medium accompanying the corrosion inhibitor 
harmonizes well with the carrying out of an acidic rinsing before the 
sheet is coated, and with the optional presence of K.sup.+, Zn.sup.2+ and 
Ni.sup.2+ ions in the atmosphere of the electroplating and coiling shop. 
It is essential for this purpose that the buffer should impose a pH not 
lower than 6.5, measured at ambient temperature (and therefore under 
inhibitor placed in aqueous solution and not yet applied to the sheet). 
The corrosion inhibitor according to the invention is an aqueous solution 
of an alkali metal nitrite at a concentration of 0.01 to 0.3 mol/l, and of 
an alkali metal phosphate at a concentration of 0.01 to 0.3 mol/l, and 
also containing phosphoric acid at a concentration such that it imposes on 
the solution a pH of between 6.9 and 7.2, measured at ambient temperature. 
It is also desirable that the molar concentrations of alkali metal ions of 
the nitrite and of alkali metals of the phosphate should not differ by 
more than 20%. By way of example, very good results have been obtained 
with a solution containing 6 g/l of sodium nitrite, 10 g/l of sodium 
phosphate Na.sub.3 PO.sub.4 .multidot.12H.sub.2 O, the quantity of 
phosphoric acid making it possible to adjust the pH to 7, and a nonionic 
wetting agent. 
Its effectiveness has been tested by applying a corrosive solution of 
chloride ions onto an unoiled sheet, by oiling the metal and subjecting it 
to cycles successively comprising 8 hours' exposure to a moist atmosphere 
and 16 hours' exposure to a dry atmosphere. After 68 hours of such a 
treatment, half as many pits are observed on the sheet when it has been 
coated with a solution of this inhibitor before the application of the 
corrosive solution. 
A nonionic or anionic wetting agent, such as a phosphoric ester, may be 
optionally added to this inhibitor solution. This overcomes the poor 
wettability of the zinc-nickel layer and does so without employing 
amine-based products in the inhibitor. It is desirable to add a foam 
suppressor together with the wetting agent. The quantities added may be, 
for example, from 1 to 5 ml/l of wetting agent and from 0.2 to 1 ml/l of 
foam suppressor. 
The application of the inhibitor fits within the following scheme: 
zinc electroplating of the sheet on one of its faces with a zinc-nickel 
alloy; 
acidic rinsing followed by rinsing of the sheet with demineralized water; 
application of the inhibitor solution; 
drying of the sheet, resulting in the evaporation of water; 
oiling of the sheet; 
coiling of the sheet and storing or despatch of the coil. 
Various methods of application of the inhibitor can be envisaged, for 
example with the aid of wringing rolls between which the sheet travels and 
whose lower parts dip in troughs containing the solution including the 
inhibitor. These wringing rolls must apply, for example, 1 to 2 ml of 
solution per m.sup.2 of sheet (on each face involved in the treatment), 
corresponding to a quantity of sodium of 2 to 6 mg/m.sup.2. It is to be 
understood that it is possible, at will, to coat both faces of the sheet 
or only the bare face. 
The use of this corrosion inhibitor is, of course, not restricted to the 
protection of sheets coated with a zinc-nickel alloy on only one face, 
which is only a preferential application, but it is applicable to the 
protection of any sheet of steel or even of other metal materials.